The Giffnock-born forward had been a long-term target for the Dons and opted to join despite interest from other Premiership clubs and teams in the EFL.
Smith joined Livingston from Hamilton Academical and spent two seasons with the club, making 40 appearances last term as they were relegated from the Scottish Premiership.
"I’m so excited to finally get it done,” Smith told club website.
“It’s taken a few weeks to get here but I’m happy to have finally arrived and buzzing to get going.
“This is a massive Club, the fans are incredible. I actually made my debut at Pittodrie, so I know what the atmosphere is like.
“I’m looking forward to working with the manager. He talked so passionately about what he is looking to achieve here. I think he will know how to get the best out of me.
“I want to bring excitement to the fans. I am direct, I can create chances and score goals. I’ve played a lot of football and hopefully I can continue my development at Aberdeen. I know I’ve got a lot more to give.”