The experienced 31-year-old former Swans captain, who's spent loan spells at Championship clubs Leeds and Yeovil, has signed until the summer.
Dons boss Derek McInnes has repeatedly said that he is keen to strengthen the defence with Clark Robertson and Andrew Considine suffering long-term injuries and cult hero Michael Hector returning to Reading after the Killie game this weekend.
But McInnes refused to rule out a Pittodrie return for Big Hec at the end of the month, who heads back to Reading mainly due to an injury crisis at the Madejski Stadium.
McInnes told RedWeb: "I've made no secret of the fact that, even if Michael does come back, we are pretty short in terms of cover in that department.
"And I'm delighted to bring in someone with Alan's experience.
"While Alan is predominantly a centre back, he can also play at full back if required, and he'll go straight into the squad for Sunday (against Kilmarnock)."