diff --git a/sigstore/_internal/oidc/ambient.py b/sigstore/_internal/oidc/ambient.py
index fdd47485c..96c0a04f1 100644
--- a/sigstore/_internal/oidc/ambient.py
+++ b/sigstore/_internal/oidc/ambient.py
@@ -45,7 +45,7 @@ def detect_credential() -> Optional[str]:
     Raises `AmbientCredentialError` if any detector fails internally (i.e.
     detects a credential, but cannot retrieve it).
     """
-    detectors: List[Callable[..., Optional[str]]] = [detect_github]
+    detectors: List[Callable[..., Optional[str]]] = [detect_github, detect_circleci]
     for detector in detectors:
         credential = detector()
         if credential is not None:
@@ -76,7 +76,7 @@ def detect_github() -> Optional[str]:
     req_url = os.getenv("ACTIONS_ID_TOKEN_REQUEST_URL")
     if not req_token or not req_url:
         raise AmbientCredentialError(
-            "GitHub: missing or insufficient OIDC token permissions?"
+            "GitHub: missing or insufficient OIDC token permissions"
         )
 
     resp = requests.get(
@@ -96,3 +96,19 @@ def detect_github() -> Optional[str]:
         return _GitHubTokenPayload(**body).value
     except Exception as e:
         raise AmbientCredentialError("GitHub: malformed or incomplete JSON") from e
+
+
+def detect_circleci() -> Optional[str]:
+    logger.debug("CircleCI: looking for OIDC credentials")
+
+    if not os.getenv("CIRCLECI"):
+        logger.debug("CircleCI: environment doesn't look right; giving up")
+        return None
+
+    token = os.getenv("CIRCLE_OIDC_TOKEN")
+    if not token:
+        raise AmbientCredentialError(
+            "CircleCI: missing or insufficient OIDC token permissions"
+        )
+
+    return token
diff --git a/test/internal/oidc/test_ambient.py b/test/internal/oidc/test_ambient.py
index 3c551e4c8..a8bbe0335 100644
--- a/test/internal/oidc/test_ambient.py
+++ b/test/internal/oidc/test_ambient.py
@@ -20,8 +20,9 @@
 
 
 def test_detect_credential_none(monkeypatch):
-    detect_github = pretend.call_recorder(lambda: None)
-    monkeypatch.setattr(ambient, "detect_github", detect_github)
+    detect_noop = pretend.call_recorder(lambda: None)
+    for detector in ["detect_github", "detect_circleci"]:
+        monkeypatch.setattr(ambient, detector, detect_noop)
     assert ambient.detect_credential() is None
 
 
@@ -32,7 +33,7 @@ def test_detect_credential(monkeypatch):
     assert ambient.detect_credential() == "fakejwt"
 
 
-def test_detect_github_bad_env(monkeypatch):
+def test_detect_github_wrong_env(monkeypatch):
     # We might actually be running in a CI, so explicitly remove this.
     monkeypatch.delenv("GITHUB_ACTIONS", raising=False)
 
@@ -56,7 +57,7 @@ def test_detect_github_bad_permissions(monkeypatch):
 
     with pytest.raises(
         ambient.AmbientCredentialError,
-        match="GitHub: missing or insufficient OIDC token permissions?",
+        match="GitHub: missing or insufficient OIDC token permissions",
     ):
         ambient.detect_github()
     assert logger.debug.calls == [
@@ -136,3 +137,21 @@ def test_detect_github(monkeypatch):
         )
     ]
     assert resp.json.calls == [pretend.call()]
+
+
+def test_detect_circleci_wrong_env(monkeypatch):
+    # We might actually be running in a CI, so explicitly remove this.
+    monkeypatch.delenv("CIRCLECI", raising=False)
+
+    assert ambient.detect_circleci() is None
+
+
+def test_detect_circleci_bad_permissions(monkeypatch):
+    monkeypatch.setenv("CIRCLECI", "true")
+    monkeypatch.delenv("CIRCLE_OIDC_TOKEN", raising=False)
+
+    with pytest.raises(
+        ambient.AmbientCredentialError,
+        match="CircleCI: missing or insufficient OIDC token permissions",
+    ):
+        ambient.detect_circleci()