@@ -509,10 +509,9 @@ def print_png(self, filename_or_obj, *args,
 
         if metadata is None:
             metadata = {}
-        metadata = {
+        default_metadata = {
             "Software":
                 f"matplotlib version{__version__}, http://matplotlib.org/",
-            **metadata,
         }
 
         FigureCanvasAgg.draw(self)
@@ -521,9 +520,13 @@ def print_png(self, filename_or_obj, *args,
             from PIL.PngImagePlugin import PngInfo
             # Only use the metadata kwarg if pnginfo is not set, because the
             # semantics of duplicate keys in pnginfo is unclear.
-            if "pnginfo" not in pil_kwargs:
+            if "pnginfo" in pil_kwargs:
+                if metadata:
+                    cbook._warn_external("'metadata' is overridden by the "
+                                         "'pnginfo' entry in 'pil_kwargs'.")
+            else:
                 pnginfo = PngInfo()
-                for k, v in metadata.items():
+                for k, v in {**default_metadata, **metadata}.items():
                     pnginfo.add_text(k, v)
                 pil_kwargs["pnginfo"] = pnginfo
             pil_kwargs.setdefault("dpi", (self.figure.dpi, self.figure.dpi))
@@ -533,8 +536,8 @@ def print_png(self, filename_or_obj, *args,
         else:
             renderer = self.get_renderer()
             with cbook.open_file_cm(filename_or_obj, "wb") as fh:
-                _png.write_png(renderer._renderer, fh,
-                               self.figure.dpi, metadata=metadata)
+                _png.write_png(renderer._renderer, fh, self.figure.dpi,
+                               metadata={**default_metadata, **metadata})
 
     def print_to_buffer(self):
         FigureCanvasAgg.draw(self)
 
@@ -1539,12 +1539,16 @@ def imsave(fname, arr, vmin=None, vmax=None, cmap=None, format=None,
             pil_shape = (rgba.shape[1], rgba.shape[0])
             image = Image.frombuffer(
                 "RGBA", pil_shape, rgba, "raw", "RGBA", 0, 1)
-            if format == "png" and metadata is not None:
+            if format == "png" and metadata:
                 # cf. backend_agg's print_png.
-                pnginfo = PngInfo()
-                for k, v in metadata.items():
-                    pnginfo.add_text(k, v)
-                pil_kwargs["pnginfo"] = pnginfo
+                if "pnginfo" in pil_kwargs:
+                    cbook._warn_external("'metadata' is overridden by the "
+                                         "'pnginfo' entry in 'pil_kwargs'.")
+                else:
+                    pnginfo = PngInfo()
+                    for k, v in metadata.items():
+                        pnginfo.add_text(k, v)
+                    pil_kwargs["pnginfo"] = pnginfo
             if format in ["jpg", "jpeg"]:
                 format = "jpeg"  # Pillow doesn't recognize "jpg".
                 color = tuple(