Can you tell me its history? Where did it come from? Is it likely a US native or native to elsewhere if you know? Does it prefer land or water?
I don't immediately recognize it as a turtle species with which I'm familiar. It looks like it may be a box turtle but not from the US, maybe Asian.
Check out this site:
http://www.asianturtle.org/They have a bunch of photos of turtles. I don't have time to peruse them all but saw a few that might be possibilities. If you look through there, maybe you'll find a match. Let us know.
Good luck!