diff --git a/pytorch_msssim/ssim.py b/pytorch_msssim/ssim.py
index 16380e2..10103b2 100644
--- a/pytorch_msssim/ssim.py
+++ b/pytorch_msssim/ssim.py
@@ -60,7 +60,8 @@ def _ssim(
     data_range: float,
     win: Tensor,
     size_average: bool = True,
-    K: Union[Tuple[float, float], List[float]] = (0.01, 0.03)
+    K: Union[Tuple[float, float], List[float]] = (0.01, 0.03),
+    mask: Optional[Tensor] = None,
 ) -> Tuple[Tensor, Tensor]:
     r""" Calculate ssim index for X and Y
 
@@ -70,6 +71,7 @@ def _ssim(
         data_range (float or int): value range of input images. (usually 1.0 or 255)
         win (torch.Tensor): 1-D gauss kernel
         size_average (bool, optional): if size_average=True, ssim of all images will be averaged as a scalar
+        mask (torch.Tensor): boolean mask same size as X and Y
 
     Returns:
         Tuple[torch.Tensor, torch.Tensor]: ssim results.
@@ -97,6 +99,10 @@ def _ssim(
     cs_map = (2 * sigma12 + C2) / (sigma1_sq + sigma2_sq + C2)  # set alpha=beta=gamma=1
     ssim_map = ((2 * mu1_mu2 + C1) / (mu1_sq + mu2_sq + C1)) * cs_map
 
+    if mask is not None:
+        ssim = torch.masked_select(ssim_map, mask).mean()
+        cs = torch.masked_select(cs_map, mask).mean()
+        return ssim, cs
     ssim_per_channel = torch.flatten(ssim_map, 2).mean(-1)
     cs = torch.flatten(cs_map, 2).mean(-1)
     return ssim_per_channel, cs
@@ -112,6 +118,7 @@ def ssim(
     win: Optional[Tensor] = None,
     K: Union[Tuple[float, float], List[float]] = (0.01, 0.03),
     nonnegative_ssim: bool = False,
+    mask: Optional[Tensor] = None,
 ) -> Tensor:
     r""" interface of ssim
     Args:
@@ -124,6 +131,7 @@ def ssim(
         win (torch.Tensor, optional): 1-D gauss kernel. if None, a new kernel will be created according to win_size and win_sigma
         K (list or tuple, optional): scalar constants (K1, K2). Try a larger K2 constant (e.g. 0.4) if you get a negative or NaN results.
         nonnegative_ssim (bool, optional): force the ssim response to be nonnegative with relu
+        mask (torch.Tensor): boolean mask same size as X and Y
 
     Returns:
         torch.Tensor: ssim results
@@ -131,9 +139,19 @@ def ssim(
     if not X.shape == Y.shape:
         raise ValueError(f"Input images should have the same dimensions, but got {X.shape} and {Y.shape}.")
 
+    if mask is not None and mask.shape != X.shape:
+        raise ValueError(f"Input mask should have the same dimensions as input images, but got {mask.shape} and {X.shape}.")
+
     for d in range(len(X.shape) - 1, 1, -1):
         X = X.squeeze(dim=d)
         Y = Y.squeeze(dim=d)
+        if mask is not None:
+            mask = mask.squeeze(dim=d)
+
+    if mask is not None:
+        assert size_average is True, "per channel ssim is not available if mask exist"
+        margin = win_size // 2
+        mask = mask[..., margin:-margin, margin:-margin]
 
     if len(X.shape) not in (4, 5):
         raise ValueError(f"Input images should be 4-d or 5-d tensors, but got {X.shape}")
@@ -151,11 +169,13 @@ def ssim(
         win = _fspecial_gauss_1d(win_size, win_sigma)
         win = win.repeat([X.shape[1]] + [1] * (len(X.shape) - 1))
 
-    ssim_per_channel, cs = _ssim(X, Y, data_range=data_range, win=win, size_average=False, K=K)
+    ssim_per_channel, cs = _ssim(X, Y, data_range=data_range, win=win, size_average=False, K=K, mask=mask)
     if nonnegative_ssim:
         ssim_per_channel = torch.relu(ssim_per_channel)
 
-    if size_average:
+    if mask is not None:
+        return ssim_per_channel
+    elif size_average:
         return ssim_per_channel.mean()
     else:
         return ssim_per_channel.mean(1)
@@ -274,7 +294,7 @@ def __init__(
         self.K = K
         self.nonnegative_ssim = nonnegative_ssim
 
-    def forward(self, X: Tensor, Y: Tensor) -> Tensor:
+    def forward(self, X: Tensor, Y: Tensor, mask: Optional[Tensor]) -> Tensor:
         return ssim(
             X,
             Y,
@@ -283,6 +303,7 @@ def forward(self, X: Tensor, Y: Tensor) -> Tensor:
             win=self.win,
             K=self.K,
             nonnegative_ssim=self.nonnegative_ssim,
+            mask=mask,
         )