diff --git a/rllib/algorithms/algorithm_config.py b/rllib/algorithms/algorithm_config.py
index 38e469d7559fe..e50b2285a4c11 100644
--- a/rllib/algorithms/algorithm_config.py
+++ b/rllib/algorithms/algorithm_config.py
@@ -368,7 +368,7 @@ def __init__(self, algo_class: Optional[type] = None):
         self.grad_clip = None
         self.grad_clip_by = "global_norm"
         # Simple logic for now: If None, use `train_batch_size`.
-        self.train_batch_size_per_learner = None
+        self._train_batch_size_per_learner = None
         self.train_batch_size = 32  # @OldAPIStack
 
         # These setting have been adopted from the original PPO batch settings:
@@ -2324,7 +2324,7 @@ def training(
                 )
             self.grad_clip_by = grad_clip_by
         if train_batch_size_per_learner is not NotProvided:
-            self.train_batch_size_per_learner = train_batch_size_per_learner
+            self._train_batch_size_per_learner = train_batch_size_per_learner
         if train_batch_size is not NotProvided:
             self.train_batch_size = train_batch_size
         if num_epochs is not NotProvided:
@@ -3763,14 +3763,26 @@ def rl_module_spec(self):
             return default_rl_module_spec
 
     @property
-    def total_train_batch_size(self):
-        if (
-            self.train_batch_size_per_learner is not None
-            and self.enable_rl_module_and_learner
-        ):
-            return self.train_batch_size_per_learner * (self.num_learners or 1)
-        else:
-            return self.train_batch_size
+    def train_batch_size_per_learner(self) -> int:
+        # If not set explicitly, try to infer the value.
+        if self._train_batch_size_per_learner is None:
+            return self.train_batch_size // (self.num_learners or 1)
+        return self._train_batch_size_per_learner
+
+    @train_batch_size_per_learner.setter
+    def train_batch_size_per_learner(self, value: int) -> None:
+        self._train_batch_size_per_learner = value
+
+    @property
+    def total_train_batch_size(self) -> int:
+        """Returns the effective total train batch size.
+
+        New API stack: `train_batch_size_per_learner` * [effective num Learners].
+
+        @OldAPIStack: User never touches `train_batch_size_per_learner` or
+        `num_learners`) -> `train_batch_size`.
+        """
+        return self.train_batch_size_per_learner * (self.num_learners or 1)
 
     # TODO: Make rollout_fragment_length as read-only property and replace the current
     #  self.rollout_fragment_length a private variable.
@@ -3905,18 +3917,11 @@ def validate_train_batch_size_vs_rollout_fragment_length(self) -> None:
         asking the user to set rollout_fragment_length to `auto` or to a matching
         value.
 
-        Also, only checks this if `train_batch_size` > 0 (DDPPO sets this
-        to -1 to auto-calculate the actual batch size later).
-
         Raises:
             ValueError: If there is a mismatch between user provided
                 `rollout_fragment_length` and `total_train_batch_size`.
         """
-        if (
-            self.rollout_fragment_length != "auto"
-            and not self.in_evaluation
-            and self.total_train_batch_size > 0
-        ):
+        if self.rollout_fragment_length != "auto" and not self.in_evaluation:
             min_batch_size = (
                 max(self.num_env_runners, 1)
                 * self.num_envs_per_env_runner
diff --git a/rllib/algorithms/appo/appo_learner.py b/rllib/algorithms/appo/appo_learner.py
index db59871b131c5..235bc823209f8 100644
--- a/rllib/algorithms/appo/appo_learner.py
+++ b/rllib/algorithms/appo/appo_learner.py
@@ -99,8 +99,7 @@ def after_gradient_based_update(self, *, timesteps: Dict[str, Any]) -> None:
                     config.target_network_update_freq
                     * config.circular_buffer_num_batches
                     * config.circular_buffer_iterations_per_batch
-                    * config.total_train_batch_size
-                    / (config.num_learners or 1)
+                    * config.train_batch_size_per_learner
                 )
             ):
                 for (