The 34 year old had been training with the Saints after leaving Easter Road last month.
Boss Derek McInnes is delighted to have bolstered his attacking options.
"When he scored against Germany, I started to fear the worst and I thought that somebody else would come and take him," admitted McInnes.
"He had a few token offers but nothing concrete.
"Credit to him - he's a top professional. There's not a bit of body fat on him.
"He's worked extremely hard to keep himself fit, he's mixed with all of the boys and you ask him to do something and he'll do it.
"He's got a great mentality and I thought he did well for us."