The 27-year-old has agreed a four-year contract at Craven Cottage with the option of a further year.
Castagne could be the first of a flurry of late arrivals at Fulham, who are considering moves for another full-back, a midfielder, a winger and a striker before the window shuts on Friday.
Castagne said: “I feel very good. It’s been a long transfer window, but finally I’m here, and I’m happy to be here. It’s a big relief. I’ve talked to the Manager and I just can’t wait to start training and playing.”
Tony Khan added: “I'm excited to welcome Timothy Castagne to Fulham!
“He's a versatile and very talented defender with great experience in the Premier League and international competition for Belgium. I'm pleased that Timothy has committed to a four year contract with us, and we believe that he'll be an excellent addition to Marco [Silva, manager]'s squad!”