Yes, it is, if you are willing to put some effort into your bubbles. Normally, a soap bubble blown in air will take whatever shape will combine the largest interior volume and the smallest surface area. For a single bubble, that would be a sphere. If you push two or more bubbles together, you will see that flat areas form where they join. If your bubble cluster is large enough, some of the central bubbles may become square.
Another way to create a single square bubble is to build a cubic frame from pipe cleaners or drinking straws, dip it into bubble solution, and you should be able to get a square bubble on the frame. It's even possible to blow a second square bubble inside the first one.
A third way to make square bubbles is by creating "armored" bubbles. This is done by blowing an ordinary bubble, and then coating its outer surface with a fine layer of extremely tiny (3 micrometer) styrene beads. The beads supply an additional force to the bubble's exterior which changes the sum of the net forces acting on the bubble, and a variety of shapes are possible, including cubes.