diff --git a/airflow-core/src/airflow/models/dagbag.py b/airflow-core/src/airflow/models/dagbag.py
index c4bd8eceea102..d36dbe5d3824f 100644
--- a/airflow-core/src/airflow/models/dagbag.py
+++ b/airflow-core/src/airflow/models/dagbag.py
@@ -18,6 +18,7 @@
 from __future__ import annotations
 
 import hashlib
+import math
 import time
 from collections.abc import MutableMapping
 from contextlib import nullcontext
@@ -62,9 +63,9 @@ class DBDagBag:
     """
     Internal class for retrieving dags from the database.
 
-    Optionally supports LRU+TTL caching when cache_size is provided.
-    The scheduler uses this without caching, while the API server can
-    enable caching via configuration.
+    Optionally caches deserialized dags. A size cap enables LRU eviction and a TTL
+    enables age-based eviction, with or without a size cap. The scheduler uses this
+    without caching, while the API server can enable caching via configuration.
 
     :meta private:
     """
@@ -79,8 +80,9 @@ def __init__(
         Initialize DBDagBag.
 
         :param load_op_links: Should the extra operator link be loaded when de-serializing the DAG?
-        :param cache_size: Size of LRU cache. If None or 0, uses unbounded dict (no eviction).
-        :param cache_ttl: Time-to-live for cache entries in seconds. If None or 0, no TTL (LRU only).
+        :param cache_size: Max cached entries; 0 or None means no size cap.
+        :param cache_ttl: Seconds until a cached entry expires. If > 0, entries are evicted by
+            age regardless of ``cache_size`` (with no size cap this gives TTL-only eviction).
         """
         self.load_op_links = load_op_links
         self._dags: MutableMapping[UUID | str, _CacheEntry] = {}
@@ -88,12 +90,13 @@ def __init__(
 
         self._revalidation_interval = conf.getint("core", "min_serialized_dag_update_interval")
 
-        # Initialize bounded cache if cache_size is provided and > 0
-        if cache_size and cache_size > 0:
-            if cache_ttl and cache_ttl > 0:
-                self._dags = TTLCache(maxsize=cache_size, ttl=cache_ttl)
-            else:
-                self._dags = LRUCache(maxsize=cache_size)
+        # Initialize a TTL cache if configured, otherwise a bounded LRU cache.
+        if cache_ttl and cache_ttl > 0:
+            maxsize = cache_size if cache_size and cache_size > 0 else math.inf
+            self._dags = TTLCache(maxsize=maxsize, ttl=cache_ttl)
+            self._use_cache = True
+        elif cache_size and cache_size > 0:
+            self._dags = LRUCache(maxsize=cache_size)
             self._use_cache = True
 
         # Lock required for bounded caches: cachetools caches are NOT thread-safe
diff --git a/airflow-core/tests/unit/models/test_dagbag.py b/airflow-core/tests/unit/models/test_dagbag.py
index 79668d4fe54f4..770cc3b6980d4 100644
--- a/airflow-core/tests/unit/models/test_dagbag.py
+++ b/airflow-core/tests/unit/models/test_dagbag.py
@@ -16,6 +16,7 @@
 # under the License.
 from __future__ import annotations
 
+import math
 import time
 from concurrent.futures import ThreadPoolExecutor
 from unittest.mock import MagicMock, patch
@@ -259,14 +260,24 @@ def test_lru_cache_enabled_with_cache_size(self):
         assert isinstance(dag_bag._dags, LRUCache)
 
     def test_ttl_cache_enabled_with_cache_size_and_ttl(self):
-        """Test that TTL cache is enabled when both cache_size and cache_ttl are provided."""
+        """Test that a bounded TTL cache is used when both cache_size and cache_ttl are provided."""
         dag_bag = DBDagBag(cache_size=10, cache_ttl=60)
         assert dag_bag._use_cache is True
         assert isinstance(dag_bag._dags, TTLCache)
+        assert dag_bag._dags.maxsize == 10
 
-    def test_zero_cache_size_uses_unbounded_dict(self):
-        """Test that cache_size=0 uses unbounded dict (same as no caching)."""
-        dag_bag = DBDagBag(cache_size=0, cache_ttl=60)
+    @pytest.mark.parametrize("cache_size", [0, None])
+    def test_ttl_only_without_size_cap(self, cache_size):
+        """Test that a positive cache_ttl with no size cap gives a TTL cache with maxsize=inf."""
+        dag_bag = DBDagBag(cache_size=cache_size, cache_ttl=60)
+        assert dag_bag._use_cache is True
+        assert isinstance(dag_bag._dags, TTLCache)
+        assert dag_bag._dags.maxsize == math.inf
+
+    @pytest.mark.parametrize("cache_ttl", [None, 0])
+    def test_zero_cache_size_uses_unbounded_dict(self, cache_ttl):
+        """Test that cache_size=0 without a TTL uses an unbounded dict (same as no caching)."""
+        dag_bag = DBDagBag(cache_size=0, cache_ttl=cache_ttl)
         assert dag_bag._use_cache is False
         assert isinstance(dag_bag._dags, dict)
 
@@ -310,6 +321,21 @@ def test_ttl_cache_expiry(self):
         with time_machine.travel("2025-01-01 00:00:02", tick=False):
             assert dag_bag._dags.get("test_version_id") is None
 
+    def test_ttl_only_evicts_by_ttl_not_size(self):
+        """An unbounded (maxsize=inf) TTL cache keeps every entry until it expires by age."""
+        dag_bag = DBDagBag(cache_size=0, cache_ttl=1)
+        assert dag_bag._dags.maxsize == math.inf
+        dag_bag._dags = TTLCache(maxsize=math.inf, ttl=1, timer=time.time)
+
+        with time_machine.travel("2025-01-01 00:00:00", tick=False):
+            for i in range(500):
+                dag_bag._dags[f"version_{i}"] = MagicMock()
+            assert len(dag_bag._dags) == 500
+
+        with time_machine.travel("2025-01-01 00:00:02", tick=False):
+            assert dag_bag._dags.get("version_0") is None
+            assert len(dag_bag._dags) == 0
+
     def test_lru_eviction(self):
         """Test that LRU eviction works when cache is full."""
         dag_bag = DBDagBag(cache_size=2)