Belén started playing basketball at Colegio Regina Mundi and later she moved to Club Ramón y Cajal from where she moved to Siglo XXI in the 2009/10 season. In the 2010 summer, Belén played the U16 European Champinoship and she averaged 6.4 points and 5.6 rebounds in 20 minutes per game.

During the 2010/11 season and being still U16, Belén Arrojo had her debut in Spanish Liga Femenina 2 and she averaged 7.7 points (90%FG) and +8 efficiency in just 12 minutes per game.

In the 2011 summer, Belén Arrojo won the Gold Medal in the U16 European Championship and with 20 points and 6 rebounds she was the best scorer in the Final against Belgium, being named in the First Team of the Championship after averaging 14.7 points, 6.8 rebounds and 2.2 assists in 22 minutes per game.

In the 2011/12 season, Belén Arrojo established herself as one of the best players i Liga Femenina and she averaged 13.1 points 5.6 rebounds and +12.6 efficiency in 27 minutes per game. Belén also won the LF2 Catalan Cup and she was the MVP with 22 points, 10 rebounds and +30 efficiency in the final.

In the 2012 summer, Belén Arrojo had a great start in the U18 European Championship and she averaged 16.6 points and 4.8 rebounds before getting injured. Still not fully recovered, Belén joined her generation´s National Team and had her first game with a huge display of 21 points and 7 rebounds in 17 minutes against Japan, to qualify her 1995 generation to the World Championship Final. One day later, Belén Arrojo won the Silver Medal (best position of Spain ever in a U17 World Championship) after losing the final against the United States despite Belén helping with 9 points, 12 rebounds and 4 assists

In the 2012/13 season, Belén played her last year at Siglo XXI and she averaged 12.3 points 6.2 rebounds and +13.1 efficiency per game.

In the 2013 summer, Belén Arrojo played the U19 World Championship and she finished in 4th position after averaging 10.7 points and 5.4 rebounds in 22 minutes per game, shining against Australia with 22 points 10 rebounds and +30 efficiency. Belén also played the U18 European Championship that summer and she won the Gold Medal and averaged 10.2 points and 7 rebounds in 24 minutes per game.

Belén Arrojo had her debut in Liga Femenina after signing with Burgos in the 2013/14 season and she averaged 5.9 points, 2.5 rebounds and +5.1 efficiency in 22 minutes per game, reaching the Queen´s Cup Semifinals and finishing the League in 6th position. Belén had great games against Ibaizabal (15 points +23 efficiency in 26 minutes) and Bembibre (15 points +17 efficiency).

In the 2014/15 season, Belén Arrojo signed with Mann Filter Zaragoza and she averaged 9.3 points and 3.6 rebounds in 29 minutes per game, shining against Gran Canaria (18 points +17 efficiency) and Conquero (13 points 7 rebounds +14 efficiency).

In the 2015 summer, Belén Arrojo won the Gold Medal in the U20 European Championship played in Spain and she also had her debut with the Spanish Senior National Team.

Belén Arrojo signed with Sedis Cadí in the 2015/16 season and she averaged 8.1 points, 4.7 rebounds, 1.4 steals and +8.5 efficiency in 23 minutes per game, qualifing in Liga Femenina 5th position. Belén had her best games against Añares Rioja (18 points), Uni Ferrol (16 points 8 rebounds 4 assists) and Campus Promete (+21 efficiency).

Belén Arrojo extended her contract with Sedis Cadí in the 2016/17 season and she averaged 9 points, 4.8 rebounds and +11 efficiency in 27 minutes per game. Belén had great games against Al-Qázeres (30 points 8 rebounds +35 efficiency), CREF (14 points 10 rebounds +23 efficiency) and Zamarat (13 points 6 rebounds 5 assists +22 efficiency in 19 minutes).

In the 2017/18 season, Belén Arrojo signed with Femení Sant Adrià and she averaged 11.2 points, 5.4 rebounds and +11.4 efficiency per game.

In September 2018, Belén Arrojo played the World Championship and won the Bronze Medal with the Spanish National Team.

Belén Arrojo signed with Perfumerías Avenida in the 2018/19 season and she averaged 4.1 points and 1.6 rebounds in 16 minutes per game in Spanish League and 1.7 points in 6 minutes per game in the Euroleague, becoming Queen´s Cup Champion and Spanish Eurocup Champion.

In the 2019/20 season, Belén Arrojo signed with Gernika and she averaged 8.1 points, 5.1 rebounds and +12 efficiency in 25 minutes per game in the Eurocup and 6.6 points (47% 3P), 4.8 rebounds and +10.7 efficiency in 21 minutes per game in Spanish League (12.1 points, 8.8 rebounds and +19.6 efficiency per 40 minutes), contributing to the excellent season of Gernika (17 wins and 5 losses).

Belén Arrojo extended with Gernika in the 2020/21 season and she averaged 6.2 points, 4.6 rebounds and +10.6 efficiency in 22 minutes per game in Spanish League and 6.3 points, 6.3 rebounds and +11.3 efficiency in 25 minutes per game in the Eurocup. In addition, she reached the Spanish Supercup Final.

Belén Arrojo continued in Gernika during the 2021/22 season and she avearged 7.3 points, 6.3 rebounds and +10.1 efficiency per game in the Eurocup and 4.6 points, 5.6 rebounds and +9.8 efficiency in 24 minutes per game in Spanish League.

In the 2022/23 season, Belén Arrojo signed with Hozono Jairis and she averaged 4.3 points, 6.6 rebounds and +9.4 efficiency per game in Spanish League.

Belén re-signed with Hozono Jairis in the 2023/24 season and she averaged 3.4 points, 4.6 rebounds and +7.2 efficiency per game in Spanish League.

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