diff --git a/snapcraft/models/project.py b/snapcraft/models/project.py
index bc7ae5d5a8..95ac883082 100644
--- a/snapcraft/models/project.py
+++ b/snapcraft/models/project.py
@@ -42,7 +42,12 @@
     Grammar,
 )
 from craft_platforms import DebianArchitecture
-from pydantic import ConfigDict, PrivateAttr, StringConstraints, error_wrappers
+from pydantic import (
+    ConfigDict,
+    PrivateAttr,
+    StringConstraints,
+    error_wrappers,
+)
 from pydantic.json_schema import (
     SkipJsonSchema,  # noqa: TC002 (typing-only-third-party-import) # pydantic needs to import types at runtime for validation
 )
diff --git a/snapcraft/services/project.py b/snapcraft/services/project.py
index 7a8ecab13c..6b44647cd9 100644
--- a/snapcraft/services/project.py
+++ b/snapcraft/services/project.py
@@ -23,7 +23,9 @@
 import craft_providers.bases
 from craft_application import ProjectService
 from craft_application.errors import CraftValidationError
+from craft_application.models.constraints import LicenseStr
 from craft_application.util import is_managed_mode
+from pydantic import TypeAdapter, ValidationError
 from typing_extensions import override
 
 from snapcraft.extensions import apply_extensions
@@ -48,8 +50,9 @@ class Project(ProjectService):
 
     __project_file_path: pathlib.Path | None = None
 
-    # Used to only issue a warning once.
+    # Used to only issue key warnings once.
     _ua_service_warning: bool = False
+    _license_spdx_warning: bool = False
 
     @staticmethod
     @override
@@ -65,10 +68,31 @@ def _app_preprocess_project(
         extract_parse_info(project)
         apply_root_packages(project)
         Project.validate_ua_services(project)
+        Project.validate_license_spdx(project)
 
     def get_parse_info(self) -> dict[str, list[str]]:
         return extract_parse_info(self.get_raw())
 
+    @classmethod
+    def validate_license_spdx(cls, project: dict[str, Any]) -> None:
+        if cls._license_spdx_warning:
+            return
+        cls._license_spdx_warning = True
+
+        if is_managed_mode():
+            return
+
+        lic = project.get("license")
+        if lic is None:
+            return
+
+        try:
+            TypeAdapter(LicenseStr).validate_python(lic)
+        except ValidationError:
+            craft_cli.emit.warning(
+                "Non-SPDX licenses are deprecated. Use SPDX license strings or 'proprietary' instead. For more information, see https://spdx.org/licenses/."
+            )
+
     @classmethod
     def validate_ua_services(cls, project: dict[str, Any]) -> None:
         """Warn if the 'ua-services' key is used for a base other than core22.
diff --git a/tests/unit/services/test_project.py b/tests/unit/services/test_project.py
index a7976d177a..4435884836 100644
--- a/tests/unit/services/test_project.py
+++ b/tests/unit/services/test_project.py
@@ -19,10 +19,12 @@
 import itertools
 import pathlib
 from typing import Any
+from unittest.mock import call
 
 import pytest
 import pytest_mock
 from craft_application.errors import CraftValidationError
+from craft_cli.pytest_plugin import RecordingEmitter
 
 from snapcraft import const
 from snapcraft.application import APP_METADATA
@@ -30,11 +32,13 @@
 
 
 @pytest.fixture(autouse=True)
-def reset_ua_service_warning():
-    """Reset the one-shot ua-service warning flag between tests."""
+def reset_warnings():
+    """Reset the one-shot warning flags between tests."""
     Project._ua_service_warning = False
+    Project._license_spdx_warning = False
     yield
     Project._ua_service_warning = False
+    Project._license_spdx_warning = False
 
 
 @pytest.mark.parametrize(
@@ -158,3 +162,56 @@ def test_no_warning_for_core22(self, emitter):
         Project.validate_ua_services(project)
 
         emitter.assert_interactions(None)
+
+
+class TestValidateLicense:
+    def test_non_spdx_deprecation_warns_once(self, emitter: RecordingEmitter) -> None:
+        project = {"license": "maybe"}
+
+        Project.validate_license_spdx(project)
+        Project.validate_license_spdx(project)
+
+        emitter.assert_warning(
+            "Non-SPDX licenses are deprecated. Use SPDX license strings or 'proprietary' instead. For more information, see https://spdx.org/licenses/."
+        )
+        # assert it was only shown once
+        assert len(emitter.interactions) == 1
+
+    def test_no_warning_in_managed_mode(
+        self, emitter: RecordingEmitter, mocker: pytest_mock.MockerFixture
+    ):
+        """Don't warn in managed-mode."""
+        mocker.patch("snapcraft.services.project.is_managed_mode", return_value=True)
+        project = {"license": "maybe"}
+
+        Project.validate_ua_services(project)
+
+        emitter.assert_interactions(None)
+
+    @pytest.mark.parametrize(
+        ("lic", "should_warn"),
+        [
+            ("MIT", False),
+            ("proprietary", False),
+            (None, False),
+            ("DemonicContract", True),
+        ],
+    )
+    def test_non_spdx_deprecation(
+        self,
+        lic: str | None,
+        should_warn: bool,
+        emitter: RecordingEmitter,
+    ) -> None:
+        project = {"license": lic}
+        Project.validate_license_spdx(project)
+
+        assert should_warn == (
+            call(
+                "warning",
+                "Non-SPDX licenses are deprecated. Use SPDX license strings or 'proprietary' instead. For more information, see https://spdx.org/licenses/.",
+            )
+            in emitter.interactions
+        )
+        # License should always remain unchanged
+        assert project.get("license") == lic