Both players go straight in to the squad for Tuesday night's Clydesdale Bank Premier League game with Falkirk.
Dons boss Mark McGhee told RedTV: "These things take longer than you want them to but both are players I know.
"I'm very pleased to have Jim Paterson and Steven MacLean here, particularly on the back of losing Lee (Miller).
"We need somebody - particularly now that Lee has gone - to link the play, to be a centre-forward that can hold the ball up for us.
"Steven MacLean's a boy that can take it in, and he's got a good goalscoring record.
"Jim Paterson, he was a player who - when he left me at Motherwell - had been our best player.
"Jim is an experienced player as well and that will be good for our young players."