Altium - How to merge polygons?
Building on the answer by Lovre Gaz, select both polygons that you want to merge by shift + click. Then right click and Polygon Actions -&gt; Combine Selected polygons.

The thing I want to add is that I had trouble getting this command to work unless I made the polygons overlap. This command didn't do anything, even if their edges were aligned to the same grid.

So before you do this command, edit your polygons so that they overlap a bit.