Courtois will complete his Real Madrid medical on Thursday and sign a six-year contract.
His departure will cost Real £35m and Mateo Kovacic, who joins Chelsea on loan.
Courtois will be replaced by Kepa Arrizabalaga between the Chelsea stick.
And the shot-stopper released a statement on Facebook regarding the transfer.
“Hello CFC Family,” he wrote. “With these few words I would like to express my heartfelt gratitude for four fantastic years tear have passed. Chelsea FC will always have a special place in my heart.
“I was only nineteen years old when the club showed a great belief in my abilities. The fact that such a big club was already following me was like a dream come true. The decision to first send me away (to Atletico Madrid) helped me to grow into a goalkeeper fit for Chelsea FC.
“When I returned, I was very happy to be part of one of the biggest teams in the world. I fulfilled a childhood dream playing in the Premier League, therefore I am really proud that we were able to win two Premier League titles, a League Cup and a FA Cup!
“A huge thank you to all my teammates and technical staff over the past four years who I shared this success and amazing moments with, both on and off the pitch. The team would never be successful without the help of all the other members who are devoted to the players and the club, such as the medical deportment, kit men and all the others who I can’t thank separately!
“Last but not least I want to thank the fans for their great support over the years. I hope you understand that being closer to my kids was considerable in my decision.
“I with you all the best in the future.”