diff --git a/pkgs/jni/CHANGELOG.md b/pkgs/jni/CHANGELOG.md
index b49ed8c78b..44a88a3f23 100644
--- a/pkgs/jni/CHANGELOG.md
+++ b/pkgs/jni/CHANGELOG.md
@@ -4,6 +4,9 @@
   and bootstrap jars [#2003](https://github.com/dart-lang/native/issues/2003)
 - Added `JObject.isInstanceOf` which checks whether a `JObject` is an instance 
   of a java class.
+- Fixed a [bug](https://github.com/dart-lang/native/issues/1908) where
+  Java interfaces implemented in on the main thread in Dart could deadlock when
+  invoked from the main thread outside the context of a Dart isolate.
 
 ## 0.14.0
 
diff --git a/pkgs/jni/java/src/main/java/com/github/dart_lang/jni/PortProxyBuilder.java b/pkgs/jni/java/src/main/java/com/github/dart_lang/jni/PortProxyBuilder.java
index 178d25115b..3622a5c489 100644
--- a/pkgs/jni/java/src/main/java/com/github/dart_lang/jni/PortProxyBuilder.java
+++ b/pkgs/jni/java/src/main/java/com/github/dart_lang/jni/PortProxyBuilder.java
@@ -38,11 +38,26 @@ private static final class DartImplementation {
 
   private boolean built = false;
   private final long isolateId;
+  private final boolean constructedOnMainThread;
   private final HashMap<String, DartImplementation> implementations = new HashMap<>();
   private final HashSet<String> asyncMethods = new HashSet<>();
 
+  private static boolean isOnMainThread() {
+    try {
+      Class<?> looper = Class.forName("android.os.Looper");
+      Method getMainLooper = looper.getMethod("getMainLooper");
+      Method getThread = looper.getMethod("getThread");
+      Thread mainThread = (Thread) getThread.invoke(getMainLooper.invoke(null));
+      return mainThread == Thread.currentThread();
+    } catch (Exception e) {
+      // Not on Android, so there is no concept of a "main" thread.
+      return false;
+    }
+  }
+
   public PortProxyBuilder(long isolateId) {
     this.isolateId = isolateId;
+    this.constructedOnMainThread = isOnMainThread();
   }
 
   private static String getDescriptor(Method method) {
@@ -121,7 +136,8 @@ private static native Object[] _invoke(
       Object proxy,
       String methodDescriptor,
       Object[] args,
-      boolean isBlocking);
+      boolean isBlocking,
+      boolean mayEnterIsolate);
 
   private static native void _cleanUp(long resultPtr);
 
@@ -139,6 +155,7 @@ public Object invoke(Object proxy, Method method, Object[] args) throws Throwabl
     DartImplementation implementation = implementations.get(method.getDeclaringClass().getName());
     String descriptor = getDescriptor(method);
     boolean isBlocking = !asyncMethods.contains(descriptor);
+    boolean mayEnterIsolate = isOnMainThread() && constructedOnMainThread;
     Object[] result =
         _invoke(
             implementation.port,
@@ -147,7 +164,8 @@ public Object invoke(Object proxy, Method method, Object[] args) throws Throwabl
             proxy,
             descriptor,
             args,
-            isBlocking);
+            isBlocking,
+            mayEnterIsolate);
     if (!isBlocking) {
       return null;
     }
diff --git a/pkgs/jni/src/dartjni.c b/pkgs/jni/src/dartjni.c
index b23dcba8a8..28bfabc97c 100644
--- a/pkgs/jni/src/dartjni.c
+++ b/pkgs/jni/src/dartjni.c
@@ -419,12 +419,14 @@ Java_com_github_dart_1lang_jni_PortProxyBuilder__1invoke(
     jobject proxy,
     jstring methodDescriptor,
     jobjectArray args,
-    jboolean isBlocking) {
+    jboolean isBlocking,
+    jboolean mayEnterIsolate) {
   CallbackResult* result = NULL;
   if (isBlocking) {
     result = (CallbackResult*)malloc(sizeof(CallbackResult));
   }
-  if (isolateId != (jlong)Dart_CurrentIsolate_DL() || !isBlocking) {
+  if ((isolateId != (jlong)Dart_CurrentIsolate_DL() && !mayEnterIsolate) ||
+      !isBlocking) {
     if (isBlocking) {
       init_lock(&result->lock);
       init_cond(&result->cond);
@@ -464,9 +466,23 @@ Java_com_github_dart_1lang_jni_PortProxyBuilder__1invoke(
       destroy_cond(&result->cond);
     }
   } else {
+    // Flutter-specific: `mayEnterIsolate` is `true` when the proxy was
+    // constructed on the main thread and is being invoked on the main thread.
+    //
+    // When the current isolate is `null`, enter the main isolate that is pinned
+    // to the main thread first before invoking the `functionPtr`.
+    assert(Dart_CurrentIsolate_DL() == NULL ||
+           Dart_CurrentIsolate_DL() == (Dart_Isolate)isolateId);
+    bool mustEnterIsolate = Dart_CurrentIsolate_DL() == NULL && mayEnterIsolate;
+    if (mustEnterIsolate) {
+      Dart_EnterIsolate_DL((Dart_Isolate)isolateId);
+    }
     result->object = ((jobject(*)(uint64_t, jobject, jobject))functionPtr)(
         port, (*env)->NewGlobalRef(env, methodDescriptor),
         (*env)->NewGlobalRef(env, args));
+    if (mustEnterIsolate) {
+      Dart_ExitIsolate_DL();
+    }
   }
   if (!isBlocking) {
     // No result is created in this case, there is nothing to clean up either.