diff --git a/singer_sdk/_singerlib/messages.py b/singer_sdk/_singerlib/messages.py
index 02c0f30f2..016e0b8b9 100644
--- a/singer_sdk/_singerlib/messages.py
+++ b/singer_sdk/_singerlib/messages.py
@@ -6,14 +6,11 @@
 import sys
 import typing as t
 from dataclasses import asdict, dataclass, field
-from datetime import timezone
+from datetime import datetime, timezone
 
 import simplejson as json
 from dateutil.parser import parse
 
-if t.TYPE_CHECKING:
-    from datetime import datetime
-
 
 class SingerMessageType(str, enum.Enum):
     """Singer specification message types."""
@@ -25,6 +22,18 @@ class SingerMessageType(str, enum.Enum):
     BATCH = "BATCH"
 
 
+def _default_encoding(obj: t.Any) -> str:  # noqa: ANN401
+    """Default JSON encoder.
+
+    Args:
+        obj: The object to encode.
+
+    Returns:
+        The encoded object.
+    """
+    return obj.isoformat(sep="T") if isinstance(obj, datetime) else str(obj)
+
+
 def exclude_null_dict(pairs: list[tuple[str, t.Any]]) -> dict[str, t.Any]:
     """Exclude null values from a dictionary.
 
@@ -211,7 +220,7 @@ def format_message(message: Message) -> str:
     Returns:
         The formatted message.
     """
-    return json.dumps(message.to_dict(), use_decimal=True, default=str)
+    return json.dumps(message.to_dict(), use_decimal=True, default=_default_encoding)
 
 
 def write_message(message: Message) -> None:
diff --git a/singer_sdk/helpers/_typing.py b/singer_sdk/helpers/_typing.py
index 81ffcc333..8b937d973 100644
--- a/singer_sdk/helpers/_typing.py
+++ b/singer_sdk/helpers/_typing.py
@@ -45,7 +45,7 @@ def to_json_compatible(val: t.Any) -> t.Any:  # noqa: ANN401
     If given a naive datetime object, pendulum automatically makes it utc
     """
     if isinstance(val, (datetime.datetime, pendulum.DateTime)):
-        return pendulum.instance(val).isoformat()
+        return pendulum.instance(val).isoformat("T")
     return val