The highly-rated 19-year-old Catalan, who can also play in central defence, will cost the London club 5 million euros, said Barcelona who have a buy-back option on the player who made his debut last season.
Barca, outlining the mechanics of the deal on their website, said they can buy Romeu back for 10 million euros after one year or for 15 million after two years.
It is not the first time the Spanish and European champions have struck a deal with a buy-back option for a product of their famed youth academy.
Last month forward Bojan Krkic joined Italian club AS Roma for 12 million euros but Barca are obliged to buy him back for 13 million in two years - effectively a loan deal.
Romeu, who began his career at Barcelona's smaller neighbours Espanyol and played mainly in Barca's B team last season, will join up with Chelsea after representing Spain at the Under-20 World Cup in Colombia which finishes on August 20.
Capable of playing a holding role in midfield, Romeu will provide Chelsea's new boss Andre Villas-Boas with options following Michael Essien's cruciate ligament injury that could sideline the Ghanaian powerhouse for six months.
