diff --git a/nibabel/tests/test_nifti1.py b/nibabel/tests/test_nifti1.py
index 5ee4fb3c1..e9f79319e 100644
--- a/nibabel/tests/test_nifti1.py
+++ b/nibabel/tests/test_nifti1.py
@@ -30,6 +30,7 @@
     Nifti1Image,
     Nifti1Pair,
     Nifti1PairHeader,
+    NiftiJSONExtension,
     data_type_codes,
     extension_codes,
     load,
@@ -1388,6 +1389,56 @@ def test_nifti_dicom_extension():
         Nifti1DicomExtension(2, 0)
 
 
+def test_json_extension(tmp_path):
+    nim = load(image_file)
+    hdr = nim.header
+    exts_container = hdr.extensions
+
+    # Test basic functionality
+    json_ext = NiftiJSONExtension('ignore', b'{"key": "value"}')
+    assert json_ext.get_content() == {'key': 'value'}
+    byte_content = json_ext._mangle(json_ext.get_content())
+    assert byte_content == b'{"key": "value"}'
+    json_obj = json_ext._unmangle(byte_content)
+    assert json_obj == {'key': 'value'}
+    size = 16 * ((len(byte_content) + 7) // 16 + 1)
+    assert json_ext.get_sizeondisk() == size
+
+    def ext_to_bytes(ext, byteswap=False):
+        bio = BytesIO()
+        ext.write_to(bio, byteswap)
+        return bio.getvalue()
+
+    # Check serialization
+    bytestring = ext_to_bytes(json_ext)
+    assert bytestring[:8] == struct.pack('<2I', size, extension_codes['ignore'])
+    assert bytestring[8:].startswith(byte_content)
+    assert len(bytestring) == size
+
+    # Save to file and read back
+    exts_container.append(json_ext)
+    nim.to_filename(tmp_path / 'test.nii')
+
+    # We used ignore, so it comes back as a Nifti1Extension
+    rt_img = Nifti1Image.from_filename(tmp_path / 'test.nii')
+    assert len(rt_img.header.extensions) == 3
+    rt_ext = rt_img.header.extensions[-1]
+    assert rt_ext.get_code() == extension_codes['ignore']
+    assert rt_ext.get_content() == byte_content
+
+    # MRS is currently the only JSON extension
+    json_ext._code = extension_codes['mrs']
+    nim.to_filename(tmp_path / 'test.nii')
+
+    # Check that the extension is read back as a NiftiJSONExtension
+    rt_img = Nifti1Image.from_filename(tmp_path / 'test.nii')
+    assert len(rt_img.header.extensions) == 3
+    rt_ext = rt_img.header.extensions[-1]
+    assert rt_ext.get_code() == extension_codes['mrs']
+    assert isinstance(rt_ext, NiftiJSONExtension)
+    assert rt_ext.get_content() == json_obj
+
+
 class TestNifti1General:
     """Test class to test nifti1 in general