Ortiz, 29, is the first player to join the SPL champions since Ally McCoist took over as manager and is a further boost after Allan McGregor and Steven Whittaker penned new contracts.
"He's a big signing," assistant manager Kenny McDowall said. "Because we are not spending millions and millions it doesn't mean the guys we are bringing in won't be up to scratch.
"Juan can play as a right midfielder and in both full-back positions but we are looking at the more forward-thinking aspects of his play."
Almeria reached the semi-finals of the Spanish Cup last season but suffered the agony of the drop, finishing bottom with 30 points.