Born in Barcelona, Catalonia, Bellerín started his club football career in Barcelona’s youth team. He moved to Arsenal in the summer of 2011, and signed his first professional contract the following year. While a youth player at Arsenal, Bellerín also helped the club to reach fourth place of the NextGen Series of 2012–13. Bellerín made his competitive debut away to West Bromwich Albion in the third round of the League Cup on 25 September 2013, coming on in the 95th minute as a substitute for Mikel Arteta in a penalty shootout victory after a 1–1 draw.

Two months later, Bellerín joined Championship club Watford on a two-month loan deal, and made his debut against Yeovil Town eight days after signing. The loan at Watford was extended until the end of the season, but he was recalled by Arsenal in February 2014.

Following injuries to Mathieu Debuchy, Calum Chambers and Nacho Monreal, Bellerín made his Champions League debut on 16 September 2014 in a 2–0 defeat away to Borussia Dortmund. He scored his first goal for Arsenal on 1 February 2015, in a 5–0 win against Aston Villa, and added a second on 4 April to open a 4–1 win over Liverpool despite also conceding a penalty kick through a foul on Raheem Sterling.

Bellerín was selected to start for Arsenal in the 2015 FA Cup Final on 30 May, helping the team keep a clean sheet in a 4–0 win over Aston Villa at Wembley Stadium.

He signed a new long-term contract before the 2015–16 season and played the entirety of Arsenal’s 1–0 win over rivals Chelsea in the 2015 FA Community Shield. He was the sole Arsenal player listed in the season’s PFA Team of the Year, and came in third place in Arsenal’s Player of the Season voting. On 21 November 2016, he signed a new long-term contract after agreeing to a deal that ties him to the club until 2022.

Having reached the semi-finals of the 2013 European Championship with the under-19 team, Bellerín made his debut for Albert Celades’s under-21 team on 30 March 2015, playing the full 90 minutes of a 4–0 friendly win over Belarus in León.

On 29 May 2016, after being named as a stand-by player for the year’s European Championship, Bellerín made his debut for the full squad by starting in a 3–1 friendly win against Bosnia and Herzegovina at the AFG Arena in St. Gallen, Switzerland. Two days later, he was chosen for the final squad after Dani Carvajal withdrew through injury. He was unused as they reached the last 16.

Bellerín became a vegan in 2017, which he credits for improving his health. In an interview in August 2019, he said that footballers have a responsibility to create awareness around environmental issues and that “It’s nice to show what we have—our cars, our watches—but good to send a more meaningful message than ‘look how cool we are’.”

Bellerín scored his first goal of Arsenal’s 2017–18 season in Arsenal’s 3–1 victory over 1. FC Köln. On 3 January 2018, Bellerín earned a draw for Arsenal against Chelsea with what BBC Sport described as ‘stunning injury-time strike’ in the 92nd minute, having earlier conceded a penalty for a foul on Eden Hazard. Bellerín played the full time match as Arsenal lost the EFL Cup by 3-0 to Manchester City at Wembley. 7 March 2018, Bellerín was ruled out of the Europa League first leg clash against Milan with an unknown return date.

On 24 September 2019, Bellerín made his return from injury as a substitute in Arsenal’s 5-0 victory against Nottingham Forest in the EFL Cup. On 1 August 2020, Bellerín was selected to start in the FA Cup Final against Chelsea, and went on to win his third winners’ medal as Arsenal won their 14th FA Cup.

Before the 2019 United Kingdom general election, Bellerin criticised Prime Minister Boris Johnson in a tweet where he encouraged young people to vote, accompanied by a hashtag #FuckBoris.

The 2019–20 Premier League was suspended in March 2020 because of the COVID-19 pandemic. When it returned in June, Bellerin pledged to plant 3,000 trees for every Arsenal victory in the remainder of the season.

Bellerín became the second-largest shareholder of EFL League Two club Forest Green Rovers in September 2020. He praised their commitments to veganism and environmentalism.