The former Celtic and Hearts striker completed his medical on Monday night after agreeing the switch to McDiarmid Park on Friday.
Beattie had looked set to join Rangers after training with the Ibrox side, but the move fell through after a number of weeks of negotiations.
He could now be set to lead the line against the Dons on Saturday - strikers Gregory Tade and Rowan Vine are both suspended - now that his future is sorted.
Beattie told Saints' official website: "I spoke to the manager and liked what he had to say about the club. He played a massive part in bringing me here.
"I already know some of the boys and I'm aware that there is a good squad already at McDiarmid.
"The manager was keen on me last season, but I went to Hearts which was the right thing for me at the time.
"I've been okay with the situation, but maybe it's not been so easy for my family, not knowing what was going to happen.
"I had other options, but I've been quite calm about the decision-making process and I'm delighted to be here.
"The club made the top six last season and there is no reason why we can't repeat that. We'll see how training goes this week, but if everything is okay I'll be happy to play against Aberdeen."