The 22-year-old, who joined the Blues from Barcelona in 2011, is now tied to Stamford Bridge until 2017.
The former Spain Under-21 international spent last season on loan at Valencia in his homeland, but returned to Chelsea this summer and played in Wednesday night's 5-0 friendly win over Wycombe.
That was Romeu's first appearance for the Blues since he suffered an anterior cruciate ligament injury at Sunderland in December 2012.
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