diff --git a/setup.cfg b/setup.cfg
index 60417f4..060a555 100644
--- a/setup.cfg
+++ b/setup.cfg
@@ -26,14 +26,6 @@ package_dir = =src
 packages =
   canonicaljson
 
-install_requires =
-    # simplejson versions before 3.14.0 had a bug with some characters
-    # (e.g. \u2028) if ensure_ascii was set to false.
-    simplejson>=3.14.0
-    # typing.Protocol was only added to the stdlib in Python 3.8
-    typing_extensions>=4.0.0; python_version < '3.8'
-
-
 [options.package_data]
 canonicaljson = py.typed
 
diff --git a/src/canonicaljson/__init__.py b/src/canonicaljson/__init__.py
index 24ed332..3861434 100644
--- a/src/canonicaljson/__init__.py
+++ b/src/canonicaljson/__init__.py
@@ -14,13 +14,8 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 import functools
-import platform
-from typing import Any, Callable, Generator, Iterator, Type, TypeVar
-
-try:
-    from typing import Protocol
-except ImportError:  # pragma: no cover
-    from typing_extensions import Protocol  # type: ignore[assignment]
+import json
+from typing import Callable, Generator, Type, TypeVar
 
 
 __version__ = "1.6.5"
@@ -63,54 +58,21 @@ def register_preserialisation_callback(
     _preprocess_for_serialisation.register(data_type, callback)
 
 
-class Encoder(Protocol):  # pragma: no cover
-    def encode(self, data: object) -> str:
-        pass
-
-    def iterencode(self, data: object) -> Iterator[str]:
-        pass
-
-    def __init__(self, *args: Any, **kwargs: Any) -> None:
-        pass
-
-
-class JsonLibrary(Protocol):  # pragma: no cover
-    @property
-    def JSONEncoder(self) -> Type[Encoder]:
-        pass
-
-
-# Declare these in the module scope, but they get configured in
-# set_json_library.
-_canonical_encoder: Encoder = None  # type: ignore[assignment]
-_pretty_encoder: Encoder = None  # type: ignore[assignment]
-
-
-def set_json_library(json_lib: JsonLibrary) -> None:
-    """
-    Set the underlying JSON library that canonicaljson uses to json_lib.
-
-    Params:
-        json_lib: The module to use for JSON encoding. Must have a
-            `JSONEncoder` property.
-    """
-    global _canonical_encoder
-    _canonical_encoder = json_lib.JSONEncoder(
-        ensure_ascii=False,
-        allow_nan=False,
-        separators=(",", ":"),
-        sort_keys=True,
-        default=_preprocess_for_serialisation,
-    )
-
-    global _pretty_encoder
-    _pretty_encoder = json_lib.JSONEncoder(
-        ensure_ascii=False,
-        allow_nan=False,
-        indent=4,
-        sort_keys=True,
-        default=_preprocess_for_serialisation,
-    )
+# Declare these once for re-use.
+_canonical_encoder = json.JSONEncoder(
+    ensure_ascii=False,
+    allow_nan=False,
+    separators=(",", ":"),
+    sort_keys=True,
+    default=_preprocess_for_serialisation,
+)
+_pretty_encoder = json.JSONEncoder(
+    ensure_ascii=False,
+    allow_nan=False,
+    indent=4,
+    sort_keys=True,
+    default=_preprocess_for_serialisation,
+)
 
 
 def encode_canonical_json(data: object) -> bytes:
@@ -152,20 +114,3 @@ def iterencode_pretty_printed_json(data: object) -> Generator[bytes, None, None]
     """
     for chunk in _pretty_encoder.iterencode(data):
         yield chunk.encode("utf-8")
-
-
-if platform.python_implementation() == "PyPy":  # pragma: no cover
-    # pypy ships with an optimised JSON encoder/decoder that is faster than
-    # simplejson's C extension.
-    import json
-else:  # pragma: no cover
-    # using simplejson rather than regular json on CPython for backwards
-    # compatibility (simplejson on Python 3.5 handles parsing of bytes while
-    # the standard library json does not).
-    #
-    # Note that it seems performance is on par or better using json from the
-    # standard library as of Python 3.7.
-    import simplejson as json  # type: ignore[no-redef]
-
-# Set the JSON library to the backwards compatible version.
-set_json_library(json)
diff --git a/tests/test_canonicaljson.py b/tests/test_canonicaljson.py
index 44422d5..eef6b18 100644
--- a/tests/test_canonicaljson.py
+++ b/tests/test_canonicaljson.py
@@ -22,12 +22,10 @@
     encode_pretty_printed_json,
     iterencode_canonical_json,
     iterencode_pretty_printed_json,
-    set_json_library,
     register_preserialisation_callback,
 )
 
 import unittest
-from unittest import mock
 
 
 class TestCanonicalJson(unittest.TestCase):
@@ -140,19 +138,6 @@ def test_invalid_float_values(self) -> None:
         with self.assertRaises(ValueError):
             encode_pretty_printed_json(nan)
 
-    def test_set_json(self) -> None:
-        """Ensure that changing the underlying JSON implementation works."""
-        mock_json = mock.Mock(spec=["JSONEncoder"])
-        mock_json.JSONEncoder.return_value.encode.return_value = "sentinel"
-        try:
-            set_json_library(mock_json)
-            self.assertEqual(encode_canonical_json({}), b"sentinel")
-        finally:
-            # Reset the JSON library to whatever was originally set.
-            from canonicaljson import json  # type: ignore[attr-defined]
-
-            set_json_library(json)
-
     def test_encode_unknown_class_raises(self) -> None:
         class C:
             pass
diff --git a/tox.ini b/tox.ini
index 63b9d58..0c166ab 100644
--- a/tox.ini
+++ b/tox.ini
@@ -33,7 +33,5 @@ commands = python -m black --check --diff src tests
 [testenv:mypy]
 deps =
     mypy==1.0
-    types-simplejson==3.17.5
     types-setuptools==57.4.14
 commands = mypy src tests
-