diff --git a/spec/fluent.ebnf b/spec/fluent.ebnf
index 13a16f2..8f04a84 100644
--- a/spec/fluent.ebnf
+++ b/spec/fluent.ebnf
@@ -119,7 +119,7 @@ indented_char       ::= text_char - "[" - "*" - "."
 special_quoted_char ::= "\""
                       | "\\"
 special_escape      ::= "\\" special_quoted_char
-unicode_escape      ::= "\\u" /[0-9a-fA-F]{4}/
+unicode_escape      ::= "\\u{" /[0-9a-fA-F]{1,6}/ "}"
 quoted_char         ::= (any_char - special_quoted_char)
                       | special_escape
                       | unicode_escape
diff --git a/syntax/grammar.mjs b/syntax/grammar.mjs
index e52f2f9..08ae138 100644
--- a/syntax/grammar.mjs
+++ b/syntax/grammar.mjs
@@ -438,8 +438,9 @@ let special_escape =
 
 let unicode_escape =
     sequence(
-        string("\\u"),
-        regex(/[0-9a-fA-F]{4}/))
+        string("\\u{"),
+        regex(/[0-9a-fA-F]{1,6}/),
+        string("}"))
     .map(join);
 
 let quoted_char =
diff --git a/test/fixtures/astral.ftl b/test/fixtures/astral.ftl
index b77e32e..7063c57 100644
--- a/test/fixtures/astral.ftl
+++ b/test/fixtures/astral.ftl
@@ -2,8 +2,8 @@ face-with-tears-of-joy = 😂
 tetragram-for-centre = 𝌆
 
 surrogates-in-text = \uD83D\uDE02
-surrogates-in-string = {"\uD83D\uDE02"}
-surrogates-in-adjacent-strings = {"\uD83D"}{"\uDE02"}
+surrogates-in-string = {"\u{D83D}\u{DE02}"}
+surrogates-in-adjacent-strings = {"\u{D83D}"}{"\u{DE02}"}
 
 emoji-in-text = A face 😂 with tears of joy.
 emoji-in-string = {"A face 😂 with tears of joy."}
diff --git a/test/fixtures/astral.json b/test/fixtures/astral.json
index 6056fb7..422f5b8 100644
--- a/test/fixtures/astral.json
+++ b/test/fixtures/astral.json
@@ -68,7 +68,7 @@
                         "type": "Placeable",
                         "expression": {
                             "type": "StringLiteral",
-                            "value": "\\uD83D\\uDE02"
+                            "value": "\\u{D83D}\\u{DE02}"
                         }
                     }
                 ]
@@ -89,14 +89,14 @@
                         "type": "Placeable",
                         "expression": {
                             "type": "StringLiteral",
-                            "value": "\\uD83D"
+                            "value": "\\u{D83D}"
                         }
                     },
                     {
                         "type": "Placeable",
                         "expression": {
                             "type": "StringLiteral",
-                            "value": "\\uDE02"
+                            "value": "\\u{DE02}"
                         }
                     }
                 ]
diff --git a/test/fixtures/escaped_characters.ftl b/test/fixtures/escaped_characters.ftl
index 5242a4b..589e5c6 100644
--- a/test/fixtures/escaped_characters.ftl
+++ b/test/fixtures/escaped_characters.ftl
@@ -2,8 +2,8 @@
 text-backslash-one = Value with \ a backslash
 text-backslash-two = Value with \\ two backslashes
 text-backslash-brace = Value with \{placeable}
-text-backslash-u = \u0041
-text-backslash-backslash-u = \\u0041
+text-backslash-u = \u{41}
+text-backslash-backslash-u = \\u{41}
 
 ## String literals
 quote-in-string = {"\""}
@@ -14,8 +14,25 @@ mismatched-quote = {"\\""}
 unknown-escape = {"\x"}
 
 ## Unicode escapes
-string-unicode-sequence = {"\u0041"}
-string-escaped-unicode = {"\\u0041"}
+string-unicode-1digit = {"\u{9}"}
+string-unicode-2digits = {"\u{09}"}
+string-unicode-3digits = {"\u{009}"}
+string-unicode-4digits = {"\u{0009}"}
+string-unicode-5digits = {"\u{00009}"}
+string-unicode-6digits = {"\u{000009}"}
+
+escape-unicode-4digits = {"\\u{41}"}
+escape-unicode-6digits = {"\\u{01F602}"}
+
+# ERROR Too few hex digits.
+string-unicode-0digits = {"\u{}"}
+# ERROR Too many hex digits.
+string-unicode-7digits = {"\U{001F602}"}
+
+# ERROR Missing opening brace.
+string-unicode-missing-open = {"\u9}"}
+# ERROR Missing closing brace.
+string-unicode-missing-close = {"\u{9"}
 
 ## Literal braces
 brace-open = An opening {"{"} brace.
diff --git a/test/fixtures/escaped_characters.json b/test/fixtures/escaped_characters.json
index 26b1974..2dab38d 100644
--- a/test/fixtures/escaped_characters.json
+++ b/test/fixtures/escaped_characters.json
@@ -80,7 +80,14 @@
                 "elements": [
                     {
                         "type": "TextElement",
-                        "value": "\\u0041"
+                        "value": "\\u"
+                    },
+                    {
+                        "type": "Placeable",
+                        "expression": {
+                            "type": "NumberLiteral",
+                            "value": "41"
+                        }
                     }
                 ]
             },
@@ -98,7 +105,14 @@
                 "elements": [
                     {
                         "type": "TextElement",
-                        "value": "\\\\u0041"
+                        "value": "\\\\u"
+                    },
+                    {
+                        "type": "Placeable",
+                        "expression": {
+                            "type": "NumberLiteral",
+                            "value": "41"
+                        }
                     }
                 ]
             },
@@ -177,7 +191,133 @@
             "type": "Message",
             "id": {
                 "type": "Identifier",
-                "name": "string-unicode-sequence"
+                "name": "string-unicode-1digit"
+            },
+            "value": {
+                "type": "Pattern",
+                "elements": [
+                    {
+                        "type": "Placeable",
+                        "expression": {
+                            "type": "StringLiteral",
+                            "value": "\\u{9}"
+                        }
+                    }
+                ]
+            },
+            "attributes": [],
+            "comment": null
+        },
+        {
+            "type": "Message",
+            "id": {
+                "type": "Identifier",
+                "name": "string-unicode-2digits"
+            },
+            "value": {
+                "type": "Pattern",
+                "elements": [
+                    {
+                        "type": "Placeable",
+                        "expression": {
+                            "type": "StringLiteral",
+                            "value": "\\u{09}"
+                        }
+                    }
+                ]
+            },
+            "attributes": [],
+            "comment": null
+        },
+        {
+            "type": "Message",
+            "id": {
+                "type": "Identifier",
+                "name": "string-unicode-3digits"
+            },
+            "value": {
+                "type": "Pattern",
+                "elements": [
+                    {
+                        "type": "Placeable",
+                        "expression": {
+                            "type": "StringLiteral",
+                            "value": "\\u{009}"
+                        }
+                    }
+                ]
+            },
+            "attributes": [],
+            "comment": null
+        },
+        {
+            "type": "Message",
+            "id": {
+                "type": "Identifier",
+                "name": "string-unicode-4digits"
+            },
+            "value": {
+                "type": "Pattern",
+                "elements": [
+                    {
+                        "type": "Placeable",
+                        "expression": {
+                            "type": "StringLiteral",
+                            "value": "\\u{0009}"
+                        }
+                    }
+                ]
+            },
+            "attributes": [],
+            "comment": null
+        },
+        {
+            "type": "Message",
+            "id": {
+                "type": "Identifier",
+                "name": "string-unicode-5digits"
+            },
+            "value": {
+                "type": "Pattern",
+                "elements": [
+                    {
+                        "type": "Placeable",
+                        "expression": {
+                            "type": "StringLiteral",
+                            "value": "\\u{00009}"
+                        }
+                    }
+                ]
+            },
+            "attributes": [],
+            "comment": null
+        },
+        {
+            "type": "Message",
+            "id": {
+                "type": "Identifier",
+                "name": "string-unicode-6digits"
+            },
+            "value": {
+                "type": "Pattern",
+                "elements": [
+                    {
+                        "type": "Placeable",
+                        "expression": {
+                            "type": "StringLiteral",
+                            "value": "\\u{000009}"
+                        }
+                    }
+                ]
+            },
+            "attributes": [],
+            "comment": null
+        },
+        {
+            "type": "Message",
+            "id": {
+                "type": "Identifier",
+                "name": "escape-unicode-4digits"
             },
             "value": {
                 "type": "Pattern",
@@ -186,7 +326,7 @@
                         "type": "Placeable",
                         "expression": {
                             "type": "StringLiteral",
-                            "value": "\\u0041"
+                            "value": "\\\\u{41}"
                         }
                     }
                 ]
@@ -198,7 +338,7 @@
             "type": "Message",
             "id": {
                 "type": "Identifier",
-                "name": "string-escaped-unicode"
+                "name": "escape-unicode-6digits"
             },
             "value": {
                 "type": "Pattern",
@@ -207,7 +347,7 @@
                         "type": "Placeable",
                         "expression": {
                             "type": "StringLiteral",
-                            "value": "\\\\u0041"
+                            "value": "\\\\u{01F602}"
                         }
                     }
                 ]
@@ -215,6 +355,42 @@
             "attributes": [],
             "comment": null
         },
+        {
+            "type": "Comment",
+            "content": "ERROR Too few hex digits."
+        },
+        {
+            "type": "Junk",
+            "annotations": [],
+            "content": "string-unicode-0digits = {\"\\u{}\"}\n"
+        },
+        {
+            "type": "Comment",
+            "content": "ERROR Too many hex digits."
+        },
+        {
+            "type": "Junk",
+            "annotations": [],
+            "content": "string-unicode-7digits = {\"\\U{001F602}\"}\n"
+        },
+        {
+            "type": "Comment",
+            "content": "ERROR Missing opening brace."
+        },
+        {
+            "type": "Junk",
+            "annotations": [],
+            "content": "string-unicode-missing-open = {\"\\u9}\"}\n"
+        },
+        {
+            "type": "Comment",
+            "content": "ERROR Missing closing brace."
+        },
+        {
+            "type": "Junk",
+            "annotations": [],
+            "content": "string-unicode-missing-close = {\"\\u{9\"}\n"
+        },
         {
             "type": "GroupComment",
             "content": "Literal braces"