Quote Originally Posted by The Wonderful Nobody
I always thought that Squall's last name was "Leonhart" because it was something he chose on his own growing up. If it was his official name, y'know, his mother's last name, then why at the beginning of the game when the guy called out the names of who had passed the field test to become SeeD, why didn't he mention Squall's last name? He said something like, "Squall... Uhm, Squall." He had said everyone else's names... Just wondering! Lol Was there anything ever mentioned about Raine's last name? Hmm...
Actually, he doesn't say "Squall... Uhm, Squall". He says "Squall... That is all."

Anyway, Leonhart being Raine's last name is the currently accepted theory, and it seems to fit the facts to me. I can't recall if it is mentioned in the game or not right now. Sorry.