Courtois spent three years on loan at Atlético Madrid from Chelsea, before becoming Chelsea's No 1 in 2014-15.
Meanwhile, Kepa will replace the Belgium worldwide in a deal that will see him become the world's most expensive goalkeeper, surpassing the £65m paid by Liverpool to Roma for Brazil goalkeeper Alisson Becker last month.
Chelsea have confirmed the signing of goalkeeper Kepa Arrizabalaga from Athletic Bilbao. It's a curious twist of fate that his departure from Stamford Bridge would pave the way for Kepa, who was so close to joining the European champions in January, to go the opposite way and replace the Belgian at Chelsea. The midfielder will join Jorginho in Chelsea's midfield who joined the Blues from Napoli.
He has spent the past two seasons in Bilbao's first team, making 53 La Liga appearances, and has one global cap.
He will train with his new Chelsea teammates for the first time on Thursday and could even make his debut in the club's opening Premier League fixture against Huddersfield Town on Saturday.
"Athletic thanks the player for his contribution during the time he has remained in our club".
But it is not just Kepa's transfer fee that will raise eyebrows, as Chelsea have agreed to pay the 23-year-old roughly £10m-a-year over seven years - taking the total value of the transfer to £141.6m.
Eden Hazard has hinted he is keen to leave Chelsea this summer as he eyes up a move to Real Madrid, but Blues officials have told Yahoo Sport that they will not entertain any offers for their number 10.