The Spain international has been a key player in Diego Simeone’s reign at Atletico, but fell down the pecking order last season as Los Rojiblancos won La Liga.
“We have concluded the signing of Saul Niguez from Atletico Madrid on a one-season loan deal with an option for a permanent transfer at the end of the season,” Chelsea said in a statement.
The 26-year-old had been linked with a move to Barcelona earlier in the window as a burden in a deal to bring Antoine Griezmann back to Atletico.
But with Griezmann on loan at Wanda Metropolitano, Saul has been cleared to leave for London in order to free up space on Atletico’s payroll.
“I am very excited to start this new challenge with Chelsea,” Saul said. “Blues fans, I’m one of you now and I can’t wait to put on the jersey, start training and see you all.”
Saul has scored 43 goals in 340 appearances for Atletico, including the one that beat Pep Guardiola’s Bayern Munich in the Champions League semi-final first leg in 2016, as Simeone’s men reached the final for the second time in two years.
His scoring prowess adds an extra dimension to the Chelsea midfielder with few open play goals far removed from the established line of Jorginho, Mateo Kovacic and N’Golo Kante.
“We welcome Saul to the Club and believe he completes our squad as we compete for top honors in five different competitions this season,” said Chelsea manager Marina Granovskaia.
“He is a proven winner, a player with a lot of experience and we know he will be very well suited to the challenges that lie ahead on all fronts over the coming year.”
The Blues won the Champions League just five months after Tuchel’s reign as coach last season and are set to face the challenge of retaining that title and winning the Premier League this season.
Saul is Chelsea’s second signing of the summer after spending a record Â£ 97.5million ($ 134million) to re-sign Romelu Lukaku from Inter Milan.