The Finn joins Celtic for an undislosed fee and has penned a five-year contract.
“It’s been on the cards for a little while and it’s nice to get it done,” the 22-year-old admitted on Celtic TV.
"I’ve been told about a project for myself and for the football club and I’m excited to join that.
“Hopefully the fans can see a lot more of me over the next couple of years. It’s a project that’s got me here and I’m really excited for it.”
On working with Hoops boss Brendan Rodgers, Sinisalo added: “It’s not often you work under great managers such as him.
“I talked to him and the way he spoke about me, how much I felt like he wanted me here, that was what really drew me to come here.
“I can’t speak highly enough of him and I’m really excited to work under him."