The 22-year-old Spaniard, who signed from Barcelona in 2011, is now tied down until 2017.
Romeu has found his first-team opportunities limited at Stamford Bridge so far, and spent last season on loan at Valencia.
He made 33 appearances in all competitions for the Spanish club and has impressed in training since his return for Chelsea’s pre-season preparations.
Romeu was influential as the Blues thrashed League Two Wycombe in their opening pre-season fixture at Adams Park on Wednesday, before being subbed on 70 minutes.
He made his Chelsea debut against Sunderland in 2011 and made 24 appearances that season before injury hampered his development.
A Spain Under-21 international with 10 caps, Romeu was part of the Under-23 squad that competed at the 2012 London Olympics.