diff --git a/CHANGELOG.rst b/CHANGELOG.rst
index fb4819c6..fe5ed9c4 100644
--- a/CHANGELOG.rst
+++ b/CHANGELOG.rst
@@ -6,6 +6,7 @@ unreleased
 
 Bug fixes:
 
+- Keep custom field mappings scoped to each ``MarshmallowPlugin`` instance.
 - Recognize ``trace`` operations defined in view docstrings, instead of
   silently dropping them (:pr:`1059`).
   Thanks :user:`binggao1230` for the PR.
diff --git a/src/apispec/ext/marshmallow/openapi.py b/src/apispec/ext/marshmallow/openapi.py
index 3663fce6..803c4a20 100644
--- a/src/apispec/ext/marshmallow/openapi.py
+++ b/src/apispec/ext/marshmallow/openapi.py
@@ -57,6 +57,7 @@ def __init__(
         schema_name_resolver,
         spec: APISpec,
     ) -> None:
+        self.field_mapping = self.field_mapping.copy()
         self.openapi_version = (
             Version(openapi_version)
             if isinstance(openapi_version, str)
diff --git a/tests/test_ext_marshmallow.py b/tests/test_ext_marshmallow.py
index fc40046e..8f303226 100644
--- a/tests/test_ext_marshmallow.py
+++ b/tests/test_ext_marshmallow.py
@@ -318,6 +318,45 @@ def test_can_use_schema_in_header(self, spec, schema):
 
 
 class TestCustomField:
+    def test_custom_field_mapping_is_scoped_to_plugin(self):
+        class CustomField(String):
+            pass
+
+        first_plugin = MarshmallowPlugin()
+        second_plugin = MarshmallowPlugin()
+        APISpec(
+            title="First",
+            version="1.0",
+            openapi_version="3.0.0",
+            plugins=[first_plugin],
+        )
+        APISpec(
+            title="Second",
+            version="1.0",
+            openapi_version="3.0.0",
+            plugins=[second_plugin],
+        )
+
+        first_plugin.map_to_openapi_type(CustomField, "integer", "int32")
+        assert first_plugin.converter.field2property(CustomField()) == {
+            "type": "integer",
+            "format": "int32",
+        }
+        assert second_plugin.converter.field2property(CustomField()) == {
+            "type": "string"
+        }
+
+        third_plugin = MarshmallowPlugin()
+        APISpec(
+            title="Third",
+            version="1.0",
+            openapi_version="3.0.0",
+            plugins=[third_plugin],
+        )
+        assert third_plugin.converter.field2property(CustomField()) == {
+            "type": "string"
+        }
+
     def test_can_use_custom_field_decorator(self, spec_fixture):
         class CustomNameA(Field):
             pass