The goalkeeper has signed a five-year contract with Scottish champions Celtic.
The Finland international, who could not dislodge Emi Martinez as no.1, went onto Instagram to thank the club.
He said: “My time (at Villa) has come to an end. I came here as a 16 year old boy and will always be grateful for the opportunity that I was given. I have watched an amazing football club develop even further and look forward to watching their success from afar now.
“A club is only as good as the people within it and Aston Villa is full of incredible people who have all helped me on my journey and I want to thank them all.
“I take a huge step on my journey now to a massive football club with incredible support and history. I want to follow in the footsteps of the incredible goalkeepers they have had and play my part in helping them win as many trophies as possible!”