diff --git a/custom/conf/app.example.ini b/custom/conf/app.example.ini
index b59ceee4f1db9..350777ab3f28a 100644
--- a/custom/conf/app.example.ini
+++ b/custom/conf/app.example.ini
@@ -451,6 +451,10 @@ INTERNAL_TOKEN=
 ;; Cache successful token hashes. API tokens are stored in the DB as pbkdf2 hashes however, this means that there is a potentially significant hashing load when there are multiple API operations.
 ;; This cache will store the successfully hashed tokens in a LRU cache as a balance between performance and security.
 ;SUCCESSFUL_TOKENS_CACHE_SIZE = 20
+;;
+;; Force users to enroll into Two-Factor Authentication. Users without 2FA have no access to any repositories.
+;ENFORCE_TWO_FACTOR_AUTH = false
+
 
 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
diff --git a/docs/content/doc/advanced/config-cheat-sheet.en-us.md b/docs/content/doc/advanced/config-cheat-sheet.en-us.md
index df1911934c885..af171c212baac 100644
--- a/docs/content/doc/advanced/config-cheat-sheet.en-us.md
+++ b/docs/content/doc/advanced/config-cheat-sheet.en-us.md
@@ -533,6 +533,7 @@ Certain queues have defaults that override the defaults set in `[queue]` (this o
   - off - do not check password complexity
 - `PASSWORD_CHECK_PWN`: **false**: Check [HaveIBeenPwned](https://haveibeenpwned.com/Passwords) to see if a password has been exposed.
 - `SUCCESSFUL_TOKENS_CACHE_SIZE`: **20**: Cache successful token hashes. API tokens are stored in the DB as pbkdf2 hashes however, this means that there is a potentially significant hashing load when there are multiple API operations. This cache will store the successfully hashed tokens in a LRU cache as a balance between performance and security.
+- `ENFORCE_TWO_FACTOR_AUTH`: **false**: Force users to enroll into Two-Factor Authentication. Users without 2FA have no access to any repositories.
 
 ## Camo (`camo`)
 
diff --git a/models/perm/access/access.go b/models/perm/access/access.go
index 7344e114a64e2..7fbb6297a83f3 100644
--- a/models/perm/access/access.go
+++ b/models/perm/access/access.go
@@ -9,11 +9,13 @@ import (
 	"context"
 	"fmt"
 
+	"code.gitea.io/gitea/models/auth"
 	"code.gitea.io/gitea/models/db"
 	"code.gitea.io/gitea/models/organization"
 	"code.gitea.io/gitea/models/perm"
 	repo_model "code.gitea.io/gitea/models/repo"
 	user_model "code.gitea.io/gitea/models/user"
+	"code.gitea.io/gitea/modules/setting"
 )
 
 // Access represents the highest access level of a user to the repository. The only access type
@@ -36,6 +38,11 @@ func accessLevel(ctx context.Context, user *user_model.User, repo *repo_model.Re
 	restricted := false
 
 	if user != nil {
+		if setting.EnforceTwoFactorAuth {
+			if twoFactor, _ := auth.GetTwoFactorByUID(user.ID); twoFactor == nil {
+				return perm.AccessModeNone, nil
+			}
+		}
 		userID = user.ID
 		restricted = user.IsRestricted
 	}
diff --git a/models/perm/access/access_test.go b/models/perm/access/access_test.go
index 7f58be4f393b5..24182ac00f241 100644
--- a/models/perm/access/access_test.go
+++ b/models/perm/access/access_test.go
@@ -13,6 +13,7 @@ import (
 	repo_model "code.gitea.io/gitea/models/repo"
 	"code.gitea.io/gitea/models/unittest"
 	user_model "code.gitea.io/gitea/models/user"
+	"code.gitea.io/gitea/modules/setting"
 
 	"github.com/stretchr/testify/assert"
 )
@@ -22,6 +23,7 @@ func TestAccessLevel(t *testing.T) {
 
 	user2 := unittest.AssertExistsAndLoadBean(t, &user_model.User{ID: 2})
 	user5 := unittest.AssertExistsAndLoadBean(t, &user_model.User{ID: 5})
+	user24 := unittest.AssertExistsAndLoadBean(t, &user_model.User{ID: 24})
 	user29 := unittest.AssertExistsAndLoadBean(t, &user_model.User{ID: 29})
 	// A public repository owned by User 2
 	repo1 := unittest.AssertExistsAndLoadBean(t, &repo_model.Repository{ID: 1})
@@ -66,6 +68,19 @@ func TestAccessLevel(t *testing.T) {
 	level, err = access_model.AccessLevel(user29, repo24)
 	assert.NoError(t, err)
 	assert.Equal(t, perm_model.AccessModeRead, level)
+
+	// test enforced two-factor authentication
+	setting.EnforceTwoFactorAuth = true
+	{
+		level, err = access_model.AccessLevel(user2, repo1)
+		assert.NoError(t, err)
+		assert.Equal(t, perm_model.AccessModeNone, level)
+
+		level, err = access_model.AccessLevel(user24, repo1)
+		assert.NoError(t, err)
+		assert.Equal(t, perm_model.AccessModeRead, level)
+	}
+	setting.EnforceTwoFactorAuth = false
 }
 
 func TestHasAccess(t *testing.T) {
@@ -73,6 +88,7 @@ func TestHasAccess(t *testing.T) {
 
 	user1 := unittest.AssertExistsAndLoadBean(t, &user_model.User{ID: 2})
 	user2 := unittest.AssertExistsAndLoadBean(t, &user_model.User{ID: 5})
+	user24 := unittest.AssertExistsAndLoadBean(t, &user_model.User{ID: 24})
 	// A public repository owned by User 2
 	repo1 := unittest.AssertExistsAndLoadBean(t, &repo_model.Repository{ID: 1})
 	assert.False(t, repo1.IsPrivate)
@@ -92,6 +108,19 @@ func TestHasAccess(t *testing.T) {
 
 	_, err = access_model.HasAccess(db.DefaultContext, user2.ID, repo2)
 	assert.NoError(t, err)
+
+	// test enforced two-factor authentication
+	setting.EnforceTwoFactorAuth = true
+	{
+		has, err = access_model.HasAccess(db.DefaultContext, user1.ID, repo1)
+		assert.NoError(t, err)
+		assert.False(t, has)
+
+		has, err = access_model.HasAccess(db.DefaultContext, user24.ID, repo1)
+		assert.NoError(t, err)
+		assert.True(t, has)
+	}
+	setting.EnforceTwoFactorAuth = false
 }
 
 func TestRepository_RecalculateAccesses(t *testing.T) {
diff --git a/models/perm/access/repo_permission.go b/models/perm/access/repo_permission.go
index 93e3bdd6d8468..6559760b30ac0 100644
--- a/models/perm/access/repo_permission.go
+++ b/models/perm/access/repo_permission.go
@@ -8,6 +8,7 @@ import (
 	"context"
 	"fmt"
 
+	"code.gitea.io/gitea/models/auth"
 	"code.gitea.io/gitea/models/db"
 	"code.gitea.io/gitea/models/organization"
 	perm_model "code.gitea.io/gitea/models/perm"
@@ -15,6 +16,7 @@ import (
 	"code.gitea.io/gitea/models/unit"
 	user_model "code.gitea.io/gitea/models/user"
 	"code.gitea.io/gitea/modules/log"
+	"code.gitea.io/gitea/modules/setting"
 )
 
 // Permission contains all the permissions related variables to a repository for a user
@@ -168,6 +170,13 @@ func GetUserRepoPermission(ctx context.Context, repo *repo_model.Repository, use
 		return
 	}
 
+	if user != nil && setting.EnforceTwoFactorAuth {
+		if twoFactor, _ := auth.GetTwoFactorByUID(user.ID); twoFactor == nil {
+			perm.AccessMode = perm_model.AccessModeNone
+			return
+		}
+	}
+
 	var is bool
 	if user != nil {
 		is, err = repo_model.IsCollaborator(ctx, repo.ID, user.ID)
diff --git a/modules/context/context.go b/modules/context/context.go
index 4b6a21b217c3b..508e48d175982 100644
--- a/modules/context/context.go
+++ b/modules/context/context.go
@@ -768,6 +768,10 @@ func Contexter(ctx context.Context) func(next http.Handler) http.Handler {
 			ctx.Data["ShowFooterBranding"] = setting.ShowFooterBranding
 			ctx.Data["ShowFooterVersion"] = setting.ShowFooterVersion
 
+			ctx.Data["ShowTwoFactorRequiredMessage"] = setting.EnforceTwoFactorAuth &&
+				ctx.Session.Get(auth.SessionKeyUID) != nil &&
+				ctx.Session.Get(auth.SessionKeyTwofaAuthed) == nil
+
 			ctx.Data["EnableSwagger"] = setting.API.EnableSwagger
 			ctx.Data["EnableOpenIDSignIn"] = setting.Service.EnableOpenIDSignIn
 			ctx.Data["DisableMigrations"] = setting.Repository.DisableMigrations
diff --git a/modules/setting/setting.go b/modules/setting/setting.go
index 043acb733d523..99e65ffc3a5d0 100644
--- a/modules/setting/setting.go
+++ b/modules/setting/setting.go
@@ -208,6 +208,7 @@ var (
 	PasswordHashAlgo                   string
 	PasswordCheckPwn                   bool
 	SuccessfulTokensCacheSize          int
+	EnforceTwoFactorAuth               bool
 
 	Camo = struct {
 		Enabled   bool
@@ -962,6 +963,7 @@ func loadFromConf(allowEmpty bool, extraConfig string) {
 	CSRFCookieHTTPOnly = sec.Key("CSRF_COOKIE_HTTP_ONLY").MustBool(true)
 	PasswordCheckPwn = sec.Key("PASSWORD_CHECK_PWN").MustBool(false)
 	SuccessfulTokensCacheSize = sec.Key("SUCCESSFUL_TOKENS_CACHE_SIZE").MustInt(20)
+	EnforceTwoFactorAuth = sec.Key("ENFORCE_TWO_FACTOR_AUTH").MustBool(false)
 
 	InternalToken = loadSecret(sec, "INTERNAL_TOKEN_URI", "INTERNAL_TOKEN")
 	if InstallLock && InternalToken == "" {
diff --git a/options/locale/locale_en-US.ini b/options/locale/locale_en-US.ini
index 1566dfc97d422..8fd41e454b1b7 100644
--- a/options/locale/locale_en-US.ini
+++ b/options/locale/locale_en-US.ini
@@ -325,6 +325,7 @@ use_scratch_code = Use a scratch code
 twofa_scratch_used = You have used your scratch code. You have been redirected to the two-factor settings page so you may remove your device enrollment or generate a new scratch code.
 twofa_passcode_incorrect = Your passcode is incorrect. If you misplaced your device, use your scratch code to sign in.
 twofa_scratch_token_incorrect = Your scratch code is incorrect.
+twofa_required = You must setup Two-Factor Authentication to get access to repositories
 login_userpass = Sign In
 login_openid = OpenID
 oauth_signup_tab = Register New Account
diff --git a/routers/web/auth/auth.go b/routers/web/auth/auth.go
index 25d70d7c47818..e24f593689f24 100644
--- a/routers/web/auth/auth.go
+++ b/routers/web/auth/auth.go
@@ -93,6 +93,11 @@ func AutoSignIn(ctx *context.Context) (bool, error) {
 	if err := ctx.Session.Set("uname", u.Name); err != nil {
 		return false, err
 	}
+	if twofa, _ := auth.GetTwoFactorByUID(u.ID); twofa != nil {
+		if err := ctx.Session.Set(auth_service.SessionKeyTwofaAuthed, true); err != nil {
+			return false, err
+		}
+	}
 	if err := ctx.Session.Release(); err != nil {
 		return false, err
 	}
@@ -313,6 +318,8 @@ func handleSignInFull(ctx *context.Context, u *user_model.User, remember, obeyRe
 		return setting.AppSubURL + "/"
 	}
 
+	isTwofaAuthed := ctx.Session.Get("twofaUid") != nil
+
 	// Delete the openid, 2fa and linkaccount data
 	_ = ctx.Session.Delete("openid_verified_uri")
 	_ = ctx.Session.Delete("openid_signin_remember")
@@ -327,6 +334,11 @@ func handleSignInFull(ctx *context.Context, u *user_model.User, remember, obeyRe
 	if err := ctx.Session.Set("uname", u.Name); err != nil {
 		log.Error("Error setting uname %s session: %v", u.Name, err)
 	}
+	if isTwofaAuthed {
+		if err := ctx.Session.Set(auth_service.SessionKeyTwofaAuthed, true); err != nil {
+			log.Error("Error setting %s session: %v", auth_service.SessionKeyTwofaAuthed, err)
+		}
+	}
 	if err := ctx.Session.Release(); err != nil {
 		log.Error("Unable to store session: %v", err)
 	}
diff --git a/routers/web/user/setting/security/2fa.go b/routers/web/user/setting/security/2fa.go
index 5fd81bae4181b..a1601fe769f12 100644
--- a/routers/web/user/setting/security/2fa.go
+++ b/routers/web/user/setting/security/2fa.go
@@ -18,6 +18,7 @@ import (
 	"code.gitea.io/gitea/modules/log"
 	"code.gitea.io/gitea/modules/setting"
 	"code.gitea.io/gitea/modules/web"
+	auth_service "code.gitea.io/gitea/services/auth"
 	"code.gitea.io/gitea/services/forms"
 
 	"github.com/pquerna/otp"
@@ -145,12 +146,21 @@ func twofaGenerateSecretAndQr(ctx *context.Context) bool {
 func EnrollTwoFactor(ctx *context.Context) {
 	ctx.Data["Title"] = ctx.Tr("settings")
 	ctx.Data["PageIsSettingsSecurity"] = true
+	ctx.Data["ShowTwoFactorRequiredMessage"] = false
 
 	t, err := auth.GetTwoFactorByUID(ctx.Doer.ID)
 	if t != nil {
 		// already enrolled - we should redirect back!
 		log.Warn("Trying to re-enroll %-v in twofa when already enrolled", ctx.Doer)
 		ctx.Flash.Error(ctx.Tr("settings.twofa_is_enrolled"))
+
+		if ctx.Session.Get(auth_service.SessionKeyTwofaAuthed) == nil {
+			// in case a 2FA user is using an old session (the session doesn't know 2FA authed),
+			// he will be navigated to this page, we should update the session status
+			_ = ctx.Session.Set(auth_service.SessionKeyTwofaAuthed, true)
+			_ = ctx.Session.Release()
+		}
+
 		ctx.Redirect(setting.AppSubURL + "/user/settings/security")
 		return
 	}
@@ -171,6 +181,7 @@ func EnrollTwoFactorPost(ctx *context.Context) {
 	form := web.GetForm(ctx).(*forms.TwoFactorAuthForm)
 	ctx.Data["Title"] = ctx.Tr("settings")
 	ctx.Data["PageIsSettingsSecurity"] = true
+	ctx.Data["ShowTwoFactorRequiredMessage"] = false
 
 	t, err := auth.GetTwoFactorByUID(ctx.Doer.ID)
 	if t != nil {
@@ -233,6 +244,9 @@ func EnrollTwoFactorPost(ctx *context.Context) {
 		// tolerate this failure - it's more important to continue
 		log.Error("Unable to delete twofaUri from the session: Error: %v", err)
 	}
+	if err := ctx.Session.Set(auth_service.SessionKeyTwofaAuthed, true); err != nil {
+		log.Error("Unable to set %s to session: Error: %v", auth_service.SessionKeyTwofaAuthed, err)
+	}
 	if err := ctx.Session.Release(); err != nil {
 		// tolerate this failure - it's more important to continue
 		log.Error("Unable to save changes to the session: %v", err)
diff --git a/services/auth/session.go b/services/auth/session.go
index 1ec94aa0af718..8f784a7b91828 100644
--- a/services/auth/session.go
+++ b/services/auth/session.go
@@ -11,6 +11,13 @@ import (
 	"code.gitea.io/gitea/modules/log"
 )
 
+// The session keys used by different packages (in the future ...)
+const (
+	SessionKeyUID         = "uid"
+	SessionKeyUname       = "uname"
+	SessionKeyTwofaAuthed = "twofaAuthed"
+)
+
 // Ensure the struct implements the interface.
 var (
 	_ Method = &Session{}
diff --git a/templates/base/alert.tmpl b/templates/base/alert.tmpl
index b8a04b89a9585..00bb7335cd836 100644
--- a/templates/base/alert.tmpl
+++ b/templates/base/alert.tmpl
@@ -18,3 +18,8 @@
 		<p>{{.Flash.WarningMsg | Str2html}}</p>
 	</div>
 {{end}}
+{{if .ShowTwoFactorRequiredMessage}}
+	<div class="ui negative message flash-error">
+		<p><a href="{{AppSubUrl}}/user/settings/security/two_factor/enroll">{{.locale.Tr "auth.twofa_required"}} &raquo;</a></p>
+	</div>
+{{end}}
diff --git a/templates/status/404.tmpl b/templates/status/404.tmpl
index 8dd4cfb8ae38d..9309336c29ca7 100644
--- a/templates/status/404.tmpl
+++ b/templates/status/404.tmpl
@@ -2,10 +2,19 @@
 <div class="page-content ui container center full-screen-width {{if .IsRepo}}repository{{end}}">
 	{{if .IsRepo}}{{template "repo/header" .}}{{end}}
 	<div class="ui container center">
+		{{if .ShowTwoFactorRequiredMessage}}
+			<div class="ui negative message flash-error">
+				<p><a href="{{AppSubUrl}}/user/settings/security/two_factor/enroll">{{.locale.Tr "auth.twofa_required"}} &raquo;</a></p>
+			</div>
+		{{end}}
 		<p style="margin-top: 100px"><img src="{{AssetUrlPrefix}}/img/404.png" alt="404"/></p>
 		<div class="ui divider"></div>
 		<br>
-		<p>{{.locale.Tr "error404" | Safe}}
+		<p>
+			{{.locale.Tr "error404" | Safe}}
+			{{/* make a clear guide to tell a anonymous user should try to sign-in to access the repository, otherwise a 404 page may confuse a user who hasn't signed-in */}}
+			{{if not .IsSigned}}<a href="{{AppSubUrl}}/user/forget_password">{{.locale.Tr "sign_in"}} &raquo;</a>{{end}}
+		</p>
 		{{if .ShowFooterVersion}}<p>{{.locale.Tr "admin.config.app_ver"}}: {{AppVer}}</p>{{end}}
 	</div>
 </div>