The 30-year-old impressed Swansea on a trial and goes into the side to ease an injury crisis in defence, with Steven Caulker and Alan Tate currently out of action.
"It's a big opportunity for me to play at Swansea," Moras said.
"The Premier League is one of the most important competitions in the world. I will try to do my best to help the team.
"I had interest and other offers from other countries, but I do not want to say who they are because it is not important. I think Swansea is the best solution for me now."
Moras has won 15 caps for the Greece national side.