I have not been following Attack on Titan past the first season of the anime, but since I currently can't delete my answer since it's been accepted, I'll provide something based on what I've gleaned from the Wiki.
From the page about the Walls, it appears that at present, there is no knowledge of any other walled community. However, when Eren finally returns to his father's basement, he discovers that humanity has not actually been wiped out. I'm not sure if this development was entirely predictable. Early on, the story of a quick construction of massive walls while humanity was under attack was already very suspicious, but I was not invested enough to think much about it. (My hunch at the time was also more of something to the effect of, "a conspiracy destroyed most of humanity.")
Old answer, based on the first season of the anime
In some of the episode openings (particularly in the first few episodes of Attack on Titan), it's stated that:
Over a hundred years ago, humanity suddenly found itself faced with a new predator. They were more powerful than humans. Humanity was immediately pushed to the brink of extinction. The survivors built three walls: Maria, Rose, and Sina.
Without any further qualification of the statement, it's pretty much implied here (and elsewhere) that the walls contain all that's left of humanity. This sense of there having been huge losses to such an extent - to the point where entire cultures probably have been obliterated - are confirmed somewhat by things like when Mikasa's kidnappers describe her and her mother as the last of the Asians.
There're probably some more explicit statements of this elsewhere, but it's been a while since I saw Attack on Titan, and this was the first place that came to mind that stated something of this sort.
If it matters to anybody, I'm taking the quote here from the Crunchyroll sub.