diff --git a/src/sentry/api/helpers/projects.py b/src/sentry/api/helpers/projects.py
new file mode 100644
index 00000000000000..a34bcbafba592b
--- /dev/null
+++ b/src/sentry/api/helpers/projects.py
@@ -0,0 +1,78 @@
+from __future__ import annotations
+
+from collections.abc import Iterable
+from typing import NamedTuple, TypeAlias
+
+from drf_spectacular.types import OpenApiTypes
+from drf_spectacular.utils import extend_schema_field
+from rest_framework import serializers
+
+from sentry.constants import ALL_ACCESS_PROJECT_ID, ALL_ACCESS_PROJECTS_SLUG
+from sentry.utils.slug import DEFAULT_SLUG_ERROR_MESSAGE, MIXED_SLUG_REGEX
+
+ProjectIdOrSlug: TypeAlias = int | str
+
+
+class ParsedProjectIdOrSlugParams(NamedTuple):
+    ids: set[int]
+    slugs: set[str]
+
+    @property
+    def has_values(self) -> bool:
+        return bool(self.ids or self.slugs)
+
+    @property
+    def has_all_projects_sentinel(self) -> bool:
+        return ALL_ACCESS_PROJECT_ID in self.ids or ALL_ACCESS_PROJECTS_SLUG in self.slugs
+
+
+def parse_id_or_slug_params(
+    values: Iterable[ProjectIdOrSlug | None],
+) -> ParsedProjectIdOrSlugParams:
+    """
+    Partition project identifier values into numeric IDs and slugs.
+
+    All-digit values and the ``-1`` all-access project sigil are treated as IDs.
+    Everything else is treated as a slug.
+    """
+    ids: set[int] = set()
+    slugs: set[str] = set()
+    for value in values:
+        if value is None or value == "":
+            continue
+        if isinstance(value, int) and not isinstance(value, bool):
+            ids.add(value)
+            continue
+
+        value_str = str(value)
+        if value_str.isdecimal() or value_str == str(ALL_ACCESS_PROJECT_ID):
+            ids.add(int(value_str))
+        else:
+            slugs.add(value_str)
+    return ParsedProjectIdOrSlugParams(ids=ids, slugs=slugs)
+
+
+@extend_schema_field(field=OpenApiTypes.STR)
+class ProjectIdOrSlugField(serializers.Field[ProjectIdOrSlug, object, ProjectIdOrSlug, object]):
+    default_error_messages = {
+        "invalid": "Expected a project ID or slug.",
+        "invalid_slug": DEFAULT_SLUG_ERROR_MESSAGE,
+    }
+
+    def to_internal_value(self, data: object) -> ProjectIdOrSlug:
+        if data is None or isinstance(data, bool):
+            self.fail("invalid")
+        if isinstance(data, int):
+            return data
+        if not isinstance(data, str) or data == "":
+            self.fail("invalid")
+        if data.isdecimal() or data == str(ALL_ACCESS_PROJECT_ID):
+            return int(data)
+        if data == ALL_ACCESS_PROJECTS_SLUG:
+            return data
+        if MIXED_SLUG_REGEX.match(data) is None:
+            self.fail("invalid_slug")
+        return data
+
+    def to_representation(self, value: ProjectIdOrSlug) -> ProjectIdOrSlug:
+        return value
diff --git a/src/sentry/api/serializers/rest_framework/project.py b/src/sentry/api/serializers/rest_framework/project.py
index 894fd132d1eb08..c97da713cbf2c1 100644
--- a/src/sentry/api/serializers/rest_framework/project.py
+++ b/src/sentry/api/serializers/rest_framework/project.py
@@ -4,6 +4,7 @@
 from drf_spectacular.utils import extend_schema_field
 from rest_framework import serializers
 
+from sentry.api.helpers.projects import ProjectIdOrSlugField
 from sentry.models.project import Project
 
 ValidationError = serializers.ValidationError
@@ -18,6 +19,8 @@ def __init__(
         The scope parameter specifies which permissions are required to access the project field.
         If multiple scopes are provided, the project can be accessed when the user is authenticated with
         any of the scopes.
+
+        If id_allowed is true, the field accepts a project ID or slug. Otherwise, it accepts slugs only.
         """
         self.scope = scope
         self.id_allowed = id_allowed
@@ -26,14 +29,18 @@ def __init__(
     def to_representation(self, value):
         return value
 
-    def to_internal_value(self, data):
+    def to_internal_value(self, data: object) -> Project:
         try:
             if self.id_allowed:
+                project_id_or_slug = ProjectIdOrSlugField().to_internal_value(data)
                 project = Project.objects.get(
-                    organization=self.context["organization"], slug__id_or_slug=data
+                    organization=self.context["organization"], slug__id_or_slug=project_id_or_slug
                 )
             else:
-                project = Project.objects.get(organization=self.context["organization"], slug=data)
+                project_slug = self._validate_slug(data)
+                project = Project.objects.get(
+                    organization=self.context["organization"], slug=project_slug
+                )
         except Project.DoesNotExist:
             raise ValidationError("Invalid project")
 
@@ -41,3 +48,11 @@ def to_internal_value(self, data):
         if not self.context["access"].has_any_project_scope(project, scopes):
             raise ValidationError("Insufficient access to project")
         return project
+
+    def _validate_slug(self, data: object) -> str:
+        if not isinstance(data, str):
+            raise ValidationError("Invalid project")
+        project_id_or_slug = ProjectIdOrSlugField().to_internal_value(data)
+        if not isinstance(project_id_or_slug, str):
+            raise ValidationError("Invalid project")
+        return project_id_or_slug
diff --git a/tests/sentry/api/helpers/test_projects.py b/tests/sentry/api/helpers/test_projects.py
new file mode 100644
index 00000000000000..b1534ce809c47c
--- /dev/null
+++ b/tests/sentry/api/helpers/test_projects.py
@@ -0,0 +1,77 @@
+import pytest
+from rest_framework import serializers
+
+from sentry.api.helpers.projects import ProjectIdOrSlugField, parse_id_or_slug_params
+
+
+class TestParseIdOrSlugParams:
+    def test_empty_input(self) -> None:
+        params = parse_id_or_slug_params([])
+        assert params.ids == set()
+        assert params.slugs == set()
+
+    def test_numeric_values(self) -> None:
+        params = parse_id_or_slug_params(["1", "2", "3"])
+        assert params.ids == {1, 2, 3}
+        assert params.slugs == set()
+
+    def test_slug_values(self) -> None:
+        params = parse_id_or_slug_params(["my-project", "another-proj"])
+        assert params.ids == set()
+        assert params.slugs == {"my-project", "another-proj"}
+
+    def test_mixed_values(self) -> None:
+        params = parse_id_or_slug_params(["1", "my-project", 42])
+        assert params.ids == {1, 42}
+        assert params.slugs == {"my-project"}
+
+    def test_empty_values_are_skipped(self) -> None:
+        params = parse_id_or_slug_params(["", None, "1", "foo"])
+        assert params.ids == {1}
+        assert params.slugs == {"foo"}
+
+    def test_all_access_numeric_sentinel_is_id(self) -> None:
+        params = parse_id_or_slug_params(["-1"])
+        assert params.ids == {-1}
+        assert params.slugs == set()
+
+    def test_negative_non_sentinel_is_slug(self) -> None:
+        params = parse_id_or_slug_params(["-2"])
+        assert params.ids == set()
+        assert params.slugs == {"-2"}
+
+    def test_all_access_sigil_is_slug(self) -> None:
+        params = parse_id_or_slug_params(["$all"])
+        assert params.ids == set()
+        assert params.slugs == {"$all"}
+
+    def test_detects_all_access_sentinels(self) -> None:
+        assert parse_id_or_slug_params(["-1"]).has_all_projects_sentinel
+        assert parse_id_or_slug_params(["$all"]).has_all_projects_sentinel
+        assert not parse_id_or_slug_params(["1", "my-project"]).has_all_projects_sentinel
+
+    def test_deduplication(self) -> None:
+        params = parse_id_or_slug_params(["1", "1", "foo", "foo"])
+        assert params.ids == {1}
+        assert params.slugs == {"foo"}
+
+
+class TestProjectIdOrSlugField:
+    def test_accepts_ids_and_slugs(self) -> None:
+        field = ProjectIdOrSlugField()
+        assert field.to_internal_value("1") == 1
+        assert field.to_internal_value(2) == 2
+        assert field.to_internal_value("-1") == -1
+        assert field.to_internal_value("my-project") == "my-project"
+
+    def test_negative_non_sentinel_string_is_slug(self) -> None:
+        field = ProjectIdOrSlugField()
+        assert field.to_internal_value("-2") == "-2"
+
+    def test_rejects_invalid_slug(self) -> None:
+        field = ProjectIdOrSlugField()
+
+        with pytest.raises(serializers.ValidationError) as error:
+            field.to_internal_value("foo bar")
+
+        assert "Enter a valid slug" in str(error.value)
diff --git a/tests/sentry/api/serializers/rest_framework/test_project.py b/tests/sentry/api/serializers/rest_framework/test_project.py
new file mode 100644
index 00000000000000..a2d36cfb9299c8
--- /dev/null
+++ b/tests/sentry/api/serializers/rest_framework/test_project.py
@@ -0,0 +1,40 @@
+from unittest.mock import Mock
+
+from rest_framework import serializers
+
+from sentry.api.serializers.rest_framework.project import ProjectField
+from sentry.testutils.cases import TestCase
+
+
+class ProjectFieldTest(TestCase):
+    def test_id_allowed_accepts_project_id(self) -> None:
+        project = self.create_project()
+        access = Mock()
+        access.has_any_project_scope.return_value = True
+
+        class ProjectSerializer(serializers.Serializer):
+            project = ProjectField(scope="project:read", id_allowed=True)
+
+        serializer = ProjectSerializer(
+            data={"project": str(project.id)},
+            context={"organization": project.organization, "access": access},
+        )
+
+        assert serializer.is_valid(), serializer.errors
+        assert serializer.validated_data["project"] == project
+
+    def test_default_rejects_project_id(self) -> None:
+        project = self.create_project()
+        access = Mock()
+        access.has_any_project_scope.return_value = True
+
+        class ProjectSerializer(serializers.Serializer):
+            project = ProjectField(scope="project:read")
+
+        serializer = ProjectSerializer(
+            data={"project": str(project.id)},
+            context={"organization": project.organization, "access": access},
+        )
+
+        assert not serializer.is_valid()
+        assert serializer.errors == {"project": ["Invalid project"]}
diff --git a/tests/sentry/models/test_project.py b/tests/sentry/models/test_project.py
index 2fee2016bf9cf1..39b515aa00a86c 100644
--- a/tests/sentry/models/test_project.py
+++ b/tests/sentry/models/test_project.py
@@ -1,7 +1,11 @@
 from collections.abc import Iterable
 from unittest.mock import MagicMock, patch
 
+import pytest
+from django.core.exceptions import ValidationError
+
 from sentry.constants import ObjectStatus
+from sentry.db.models.fields.slug import SentrySlugField
 from sentry.deletions.models.scheduleddeletion import CellScheduledDeletion
 from sentry.deletions.tasks.hybrid_cloud import schedule_hybrid_cloud_foreign_key_jobs_control
 from sentry.grouping.grouptype import ErrorGroupType
@@ -48,6 +52,13 @@
 
 
 class ProjectTest(APITestCase, TestCase):
+    def test_slug_rejects_numeric_values(self) -> None:
+        slug_field = Project._meta.get_field("slug")
+
+        assert isinstance(slug_field, SentrySlugField)
+        with pytest.raises(ValidationError):
+            slug_field.run_validators("123")
+
     def test_member_set_simple(self) -> None:
         user = self.create_user()
         org = self.create_organization(owner=user)