Born in Ulldecona, Tarragona, Catalonia, Romeu started his career at local CF Ulldecona. He arrived in FC Barcelona’s youth academy in 2004, from neighbours RCD Espanyol, and progressed through the club’s youth academy until breaking into the reserve team. Manager Pep Guardiola handed him his first appearance for the senior side in a friendly against Kuwaiti team Kazma Sporting Club, and subsequently included him in the squad for the 2009 FIFA Club World Cup which took place in Abu Dhabi.

He participated in the 2008 UEFA European Under-17 Football Championship with Spain, playing four matches and helping his country to eventual victory in the tournament.

After making his professional début for Barça in 2009, Romeu became a mainstay in the under-19 and under-20 national teams, featuring heavily in the 2010 UEFA Under-19 Championship elite qualification round.

Romeu was included in the nineteen-man Barcelona squad on 13 August 2010, due to the need to rest some of Barça’s Spanish internationals. He was to face Sevilla in the first leg of the season’s Spanish Supercup. He made his official competitive debut in that game, playing the full ninety minutes in the 1–3 away loss.

Romeu made his first La Liga appearance on 15 May 2011, playing the last ten minutes of a 0–0 home draw against Deportivo de La Coruña in the place of another club youth graduate, Jonathan dos Santos.

Reports began to link Romeu with a move to Chelsea on 22 July 2011. The following day he agreed terms for his transfer, pending a medical or personal terms as he was away on international duty at the 2011 FIFA U-20 World Cup. On 4 August the English club announced the completion of the transfer, with the player signing a four-year contract in a deal worth €5 million – the deal also gave Barcelona first refusal via a buy back option should Chelsea deem him surplus to requirements, seeing Barcelona pay €10 million in 2012 or €15 million in 2013 to bring him back to the Camp Nou; he was given the number 6 shirt for the new campaign.

Romeu made his official debut for Chelsea on 10 September 2011, being brought on as a substitute in the 79th minute of the 2–1 away win against Sunderland. He received his first start on the 21st, in a 0–0 draw (penalty shootout win) against Fulham for the season’s Football League Cup.

Romeu’s first UEFA Champions League appearance came on 19 October 2011, playing the full 90 minutes in a 5–0 group stage home win against K.R.C. Genk. His first start in the Premier League was against Wolverhampton Wanderers on 26 November, putting in a solid performance in a 3–0 home win.

In December 2011, Chelsea issued a statement regarding the deal surrounding Romeu’s buy back clause with Barcelona. The statement highlighted that the Blaugrana did not have an automatic buy back clause as first suggested, only having first refusal on any deal if the Blues wanted to sell him in the future.

On 25 September 2012, Romeu scored his first goal for Chelsea, taking a penalty in a 6–0 win against Wolves at Stamford Bridge for the season’s League Cup. Manager Roberto Di Matteo stated that the departures of Yossi Benayoun, Michael Essien and Raul Meireles would mean more playing opportunities for the Spaniard; however, on 11 December, he suffered a knee injury in a game against Sunderland, being sidelined for the rest of the season.

On 12 July 2013, Romeu returned to Spain and joined Valencia in a season-long loan deal. He made his debut for the club on 15 September, in a 3–1 defeat at Real Betis. In February, during training Romeu suffered another knee injury that kept him out of action for a month. During his time with Valencia, Romeu played a total of 18 games.

On 17 July 2014, Romeu signed a new contract with Chelsea, keeping him at the club until 2017. On 4 August, however, he was loaned to VfB Stuttgart from Germany. On 24 August 2014, Romeu made his debut for Stuttgart against Borussia Mönchengladbach which ended in a 1–1 draw.

On 13 August 2015, Romeu joined Southampton for £5 million on a three-year deal. He made his debut two days later, as a half-time substitute for Dušan Tadić in a 3–0 home defeat to Everton, earning a booking after three minutes for a foul on James McCarthy. He scored his first goal for the Saints – and first in a professional league – on 5 December, equalising for a 1–1 draw against Aston Villa at St Mary’s Stadium.

After missing only one league game all season for Claude Puel’s Saints, Romeu signed a new four-and-a-half-year deal on 24 January 2017. He was voted the club’s Player of the Season, at the end of a campaign that saw them reach the EFL Cup Final.

In March 2019, Romeu was called up by Catalonia for a friendly against Venezuela.