diff --git a/packages/smithy-json/.changes/next-release/smithy-json-enhancement-37b9c0663ce84096905fd484478ce94d.json b/packages/smithy-json/.changes/next-release/smithy-json-enhancement-37b9c0663ce84096905fd484478ce94d.json
new file mode 100644
index 000000000..83454f1b1
--- /dev/null
+++ b/packages/smithy-json/.changes/next-release/smithy-json-enhancement-37b9c0663ce84096905fd484478ce94d.json
@@ -0,0 +1,4 @@
+{
+  "type": "enhancement",
+  "description": "Fixed string serialization to escape all control characters (U+0000-U+001F) per [RFC 8259](https://www.rfc-editor.org/rfc/rfc8259#section-7), preventing invalid JSON output for multiline and other control-character-containing strings. ([#647](https://github.com/smithy-lang/smithy-python/pull/647))"
+}
\ No newline at end of file
diff --git a/packages/smithy-json/src/smithy_json/_private/serializers.py b/packages/smithy-json/src/smithy_json/_private/serializers.py
index c1cd3df70..7146923e4 100644
--- a/packages/smithy-json/src/smithy_json/_private/serializers.py
+++ b/packages/smithy-json/src/smithy_json/_private/serializers.py
@@ -1,6 +1,7 @@
 #  Copyright Amazon.com, Inc. or its affiliates. All Rights Reserved.
 #  SPDX-License-Identifier: Apache-2.0
 
+import re
 from base64 import b64encode
 from collections.abc import Callable, Mapping, Sequence
 from contextlib import AbstractContextManager
@@ -27,6 +28,30 @@
 _INF: float = float("inf")
 _NEG_INF: float = float("-inf")
 
+# RFC 8259 §7: All control characters U+0000 through U+001F MUST be escaped.
+_ESCAPE_MAP: dict[str, str] = {
+    "\\": "\\\\",
+    '"': '\\"',
+    "\n": "\\n",
+    "\r": "\\r",
+    "\t": "\\t",
+    "\b": "\\b",
+    "\f": "\\f",
+}
+_CHARS_TO_ESCAPE = re.compile(r'[\\"\x00-\x1f]')
+
+
+def _escape_char(match: re.Match[str]) -> str:
+    c = match.group()
+    if c in _ESCAPE_MAP:
+        return _ESCAPE_MAP[c]
+    return f"\\u{ord(c):04x}"
+
+
+def _escape_string(value: str) -> str:
+    """Escape a string value per RFC 8259 §7."""
+    return _CHARS_TO_ESCAPE.sub(_escape_char, value)
+
 
 class JSONShapeSerializer(ShapeSerializer):
     def __init__(self, sink: BytesWriter, settings: JSONSettings) -> None:
@@ -271,9 +296,7 @@ def write_document_value(
 
     def write_string(self, value: str) -> None:
         self._sink.write(b'"')
-        self._sink.write(
-            value.replace("\\", "\\\\").replace('"', '\\"').encode("utf-8")
-        )
+        self._sink.write(_escape_string(value).encode("utf-8"))
         self._sink.write(b'"')
 
     def write_int(self, value: int) -> None:
diff --git a/packages/smithy-json/tests/unit/__init__.py b/packages/smithy-json/tests/unit/__init__.py
index 294dc5457..8e666cdb5 100644
--- a/packages/smithy-json/tests/unit/__init__.py
+++ b/packages/smithy-json/tests/unit/__init__.py
@@ -349,6 +349,12 @@ def _read_optional_map(k: str, d: ShapeDeserializer):
     (Decimal("1.1"), b"1.1"),
     (b"foo", b'"Zm9v"'),
     ("foo", b'"foo"'),
+    # RFC 8259 §7: control characters must be escaped
+    ("line 1\nline 2", b'"line 1\\nline 2"'),
+    ("col 1\tcol 2", b'"col 1\\tcol 2"'),
+    ("a\rb", b'"a\\rb"'),
+    ("a\\b", b'"a\\\\b"'),
+    ('a"b', b'"a\\"b"'),
     (datetime(2024, 5, 15, tzinfo=UTC), b'"2024-05-15T00:00:00Z"'),
     (None, b"null"),
     (["foo"], b'["foo"]'),
diff --git a/packages/smithy-json/tests/unit/test_serializers.py b/packages/smithy-json/tests/unit/test_serializers.py
index 552c23a0b..fd04bb7a8 100644
--- a/packages/smithy-json/tests/unit/test_serializers.py
+++ b/packages/smithy-json/tests/unit/test_serializers.py
@@ -1,5 +1,6 @@
 # Copyright Amazon.com, Inc. or its affiliates. All Rights Reserved.
 # SPDX-License-Identifier: Apache-2.0
+import json
 from datetime import datetime
 from decimal import Decimal
 from io import BytesIO
@@ -82,3 +83,59 @@ def test_json_serializer(given: Any, expected: bytes) -> None:
     sink.seek(0)
     actual = sink.read()
     assert actual == expected
+
+
+def _serialize_string(value: str) -> bytes:
+    """Serialize a string value through the JSON codec and return raw bytes."""
+    sink = BytesIO()
+    serializer = JSONCodec().create_serializer(sink)
+    serializer.write_string(STRING, value)
+    serializer.flush()
+    sink.seek(0)
+    return sink.read()
+
+
+class TestStringControlCharEscaping:
+    """RFC 8259 §7: All control characters U+0000-U+001F must be escaped."""
+
+    @pytest.mark.parametrize(
+        "char, escaped",
+        [
+            ("\n", "\\n"),
+            ("\r", "\\r"),
+            ("\t", "\\t"),
+            ("\b", "\\b"),
+            ("\f", "\\f"),
+        ],
+    )
+    def test_named_control_chars(self, char: str, escaped: str) -> None:
+        result = _serialize_string(f"a{char}b")
+        assert result == f'"a{escaped}b"'.encode()
+
+    def test_all_control_chars_produce_valid_json(self) -> None:
+        """Every U+0000-U+001F character must be escaped so output is valid JSON."""
+        for cp in range(0x20):
+            value = f"before{chr(cp)}after"
+            raw = _serialize_string(value)
+            # Must parse as valid JSON
+            parsed = json.loads(raw)
+            assert parsed == value, f"Round-trip failed for U+{cp:04X}"
+
+    def test_null_byte(self) -> None:
+        result = _serialize_string("a\x00b")
+        assert result == b'"a\\u0000b"'
+
+    def test_mixed_escapes(self) -> None:
+        result = _serialize_string('line 1\nline 2\t"quoted"\r\n')
+        assert result == b'"line 1\\nline 2\\t\\"quoted\\"\\r\\n"'
+
+    def test_existing_backslash_and_quote_still_escaped(self) -> None:
+        result = _serialize_string('a\\b"c')
+        assert result == b'"a\\\\b\\"c"'
+
+    def test_serialized_output_is_valid_json(self) -> None:
+        """Realistic multi-line prompt string produces valid JSON."""
+        value = "System: You are helpful.\nUser: Hello\nAssistant:"
+        raw = _serialize_string(value)
+        parsed = json.loads(raw)
+        assert parsed == value