The 23-year-old has signed a deal that will tie him to Pittodrie until 2018, with a transfer fee believed to be in the regioon of £300,000.
Aberdeen gaffer Derek McInnes said: "I think this is the sort of signing that a club like Aberdeen should be making.
"He's a good young player who is still improving all the time and I think in our team he will go and show how good a player he can be."
@kennymclean66 @AberdeenFC Welcome aboard min!
— Aberdeen Mad ☆☆ (@aberdeenmad) February 2, 2015
McLean is a product of St Mirren's youth system and signed a two-year contract extension in July.
The Scotland Under-21 cap has made 29 appearances this season for the Buddies, scoring seven goals.
A club statement on the St Mirren website said: "When Kenny signed a two-year contract extension with the club last summer he put his trust in St Mirren and agreements were made.
"St Mirren FC, on receipt of the offer from Aberdeen FC, have therefore honoured our side of the agreement as Kenny was keen to make this move.
"He kept his side of the agreement with his performances for the club this season."