diff --git a/homeassistant/package_constraints.txt b/homeassistant/package_constraints.txt
index 30cfabe0a4fd..66456791d4a8 100644
--- a/homeassistant/package_constraints.txt
+++ b/homeassistant/package_constraints.txt
@@ -48,7 +48,7 @@ Jinja2==3.1.6
 lru-dict==1.4.1
 mutagen==1.48.1
 openai==2.45.0
-orjson==3.12.0
+orjson==3.11.9
 packaging>=23.1
 paho-mqtt==2.1.0
 Pillow==12.3.0
diff --git a/pyproject.toml b/pyproject.toml
index 85e74c8facf1..95561ae862df 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -61,7 +61,10 @@ dependencies = [
   "Pillow==12.3.0",
   "propcache==0.5.2",
   "pyOpenSSL==26.2.0",
-  "orjson==3.12.0",
+  # Do not bump orjson until the very aggressive buffer resizing introduced in
+  # 3.12.0, which causes memory use to balloon, has been adjusted or a workaround
+  # is available.
+  "orjson==3.11.9",
   "packaging>=23.1",
   "psutil-home-assistant==0.0.1",
   "python-slugify==8.0.4",
diff --git a/requirements.txt b/requirements.txt
index 87ed11507892..88cee4233102 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -35,7 +35,7 @@ infrared-protocols==10.0.0
 Jinja2==3.1.6
 lru-dict==1.4.1
 mutagen==1.48.1
-orjson==3.12.0
+orjson==3.11.9
 packaging>=23.1
 Pillow==12.3.0
 probatio==0.11.4
diff --git a/tests/helpers/test_storage.py b/tests/helpers/test_storage.py
index 60daa8d8ceea..249dab632fe5 100644
--- a/tests/helpers/test_storage.py
+++ b/tests/helpers/test_storage.py
@@ -891,7 +891,7 @@ async def test_loading_corrupt_core_file(
         assert issue_entry.translation_placeholders["storage_key"] == storage_key
         assert issue_entry.issue_domain == HOMEASSISTANT_DOMAIN
         assert (
-            "unexpected character, expected a JSON value: line 1 column 1 (char 0)"
+            "unexpected character: line 1 column 1 (char 0)"
             in issue_entry.translation_placeholders["error"]
         )