The Dane moves to Craven Cottage for £30m. He has signed a five-year contract with a 12 month option.
Speaking to FFCtv, Andersen said: “It’s really nice to be here. Feels a little bit like coming home, so it’s a good feeling.
Joachim Andersen signs for Fulham
“Of course, I know the Club, I know how Fulham are playing, I know many players, I know some staff, so I didn’t need anyone to convince me because I knew a lot of things already.
“I had some good talks with the Manager. I know the playing style he likes to play, so it was just a decision I needed to make myself, with my wife and family, and see what’s the best for me, and I think this is a good step for my career.”
Fulham chief Tony Khan added: "I'm thrilled to welcome Joachim Andersen back to Fulham, but this time as a permanent move! Joachim is a formidable and experienced defender, he proved to be a steady leader for us during his previous loan, and I know that he's excited to join us again!"