Magennis is Buddies boss Danny Lennon's third signing of the January window after landing the Northern Ireland international's former Pittodrie team-mate Gregg Wylde on a free and Newcastle youngster Adam Campbell on a six-month loan.
The 23-year-old former Cardiff player has started just one match all season for Derek McInnes' side, making the rest of his 19 appearances from the bench.
But the arrival of former Inverness striker Adam Rooney pushed him further down the Dons' pecking order.
Lennon said: "We are delighted to have agreed a loan deal with Aberdeen for Josh Magennis until the end of the season.
"Josh has pace and strength and will add a few other dimensions up top for us. He has scored some notable goals at this level and he has the potential to do really well for us.
"He is also accustomed to playing a few different positions, which may come in handy for us.
"We were keen to take him permanently, but Derek McInnes wasn't keen to let him go, as he thinks the lad has great potential. We do too.
"Josh comes here with a point to prove that he deserves more starts at this level and we want to harness that hunger and desire here."
Lennon has, however, ruled out a deal for former Rangers striker Nacho Novo, 34, who has been training with St Mirren after leaving Morton last month.
He said: "A club our size needs to look at all options out there, as competition for players is fierce during the window.
"We brought Nacho Novo in for a chat, as he had formed a lucrative partnership with big Thomo (Steven Thompson) in the past and has scored a lot of goals in Scotland. We are no longer pursuing this avenue."