Gabriel Diallo (born September 24, 2001 in Montréal, Québec, Canada) is a Canadian professional tennis player. He reached a career-high ATP singles ranking of No. 146 on May 22, 2023 and a career-high ATP doubles ranking of No. 608 on July 15, 2019.
In 2019, Diallo advanced to his first professional doubles final at the ITF 15K in East Lansing, United States. In 2021, he reached his first professional singles final at the ITF 15K in Champaign, United States. The same year, he reached a second ITF singles final, this time at the ITF 15K in East Lansing. In 2022, Diallo captured his first singles title, defeating Andres Martin in straight sets at the ITF 25K in East Lansing. Also in 2022, he reached the qualifying last round at the ATP Masters 1000 National Bank Open as a wildcard after defeating the fourth seed and then world No. 59 James Duckworth in the first round. He was forced to retire in his next match against Hugo Gaston with an injury. In 2022, Diallo captured his first ATP Challenger title thanks to a victory over Shang Juncheng at the 80 tournament in Granby. Also in 2022, he reached his second ATP Challenger final at the 80 Series in Fairfield where he lost to Michael Mmoh. He was a member of the University of Kentucky tennis team from 2019 to 2022. After the 2022 season, he decided to forgo his remaining eligibility at the University of Kentucky and turn pro. In 2023, he won his second ITF singles title at the ITF 25K in Montréal.
Diallo was born in Montréal, Québec to an African father and an Ukrainian mother. He started playing tennis at age 9. He trained at the Club Avantage as a member of the Académie de Tennis Hérisset-Bordeleau (now Académie de tennis Aliassime) in Québec City with the father of compatriot Félix Auger-Aliassime, Sam.