Si Jiahui (Chinese: 斯佳辉 sí-tɕjá-xwéɪ; born 11 July 2002) is a Chinese professional snooker player. After training at the Wiraka Billiard Academy in Foshan, he moved to the United Kingdom aged 16 and earned a two-year tour card through the 2019 Q School for the 2019–20 and 2020–21 seasons. He lost his tour card after ending the 2020–21 season outside the top 64 in the world rankings, and competed as an amateur during the 2021–22 season, during which he defeated Shaun Murphy 6–5 in the first round of the 2021 UK Championship. After rejoining the professional tour at the start of the 2022–23 season, he reached his first ranking quarter-final at the 2022 European Masters.
At the 2023 World Snooker Championship, Si won three qualifying matches to reach the tournament's final stages at the Crucible Theatre for the first time. He then defeated Murphy, Robert Milkins, and Anthony McGill as he progressed to the semi-finals, becoming the first Crucible debutant to reach the last four since Andy Hicks in 1995. Although he led 14–5 in the semi-final, he lost 15–17 to the eventual winner Luca Brecel; this nine-frame deficit was the largest lead overturned in the history of the World Championship. He advanced from 80th to 36th in the world rankings after the tournament. Si reached the first ranking final of his career at the following season's 2024 German Masters, but lost 5–10 to Judd Trump.
Si achieved his first maximum break in professional competition in his 6–2 win against Judd Trump in the semi-final of the 2024 Wuhan Open. In his second ranking final, Si lost against Xiao Guodong.
Si received a two-year professional tour card through the 2019 Q School for the 2019–20 and 2020–21 seasons. However, he was relegated from the World Snooker Tour after finishing the 2020–21 season 92nd in the snooker world rankings, below the cut-off at 64th position.
Si spent the 2021–22 season competing as an amateur, but earned top-up places in ranking tournaments through his high placement in the 2021 Q School Order of Merit. At the 2021 UK Championship, he defeated former world champion Shaun Murphy 6–5 in the first round. In a BBC radio interview after the match, Murphy objected to amateur players featuring in professional events, saying that he had "lost to someone who shouldn’t even be in the building". The World Snooker Tour issued a statement disagreeing with these remarks, and Murphy subsequently apologised for "taking the shine off" Si's win.
Towards the end of the season, Si won the 2022 World Snooker Federation Open amateur event, defeating Lee Stephens 5–0 in the final. This win earned him a two-year professional tour card for the 2022–23 and 2023–24 seasons. At the 2022 European Masters, he defeated Anthony McGill and Daniel Wells to reach his first quarter-final at a ranking event, which he lost 2–5 to Kyren Wilson.
Ranked 80th in the world rankings, he qualified for the main stage of the 2023 World Snooker Championship after defeating Florian Nüßle 10–7, Tom Ford 10–5, and Jordan Brown 10–7 in the qualifying rounds. In the first round at the Crucible Theatre, Si defeated Murphy 10–9. On this occasion, Murphy commented on how well Si had played, saying he had been "fabulous". Si defeated Robert Milkins 13–7 in the second round and McGill 13–12 in the quarter-finals, becoming the first Crucible debutant to reach the World Championship semi-finals since Andy Hicks in 1995 and the youngest player to do so since Ronnie O'Sullivan in 1996. Si lost 15–17 to Luca Brecel in the semi-finals after Brecel, from 5–14 down, won 12 of the last 13 frames. Si advanced from 80th to 36th in the world rankings as a result of his performance at the tournament.
Si reached the first ranking final of his career at the following season's 2024 German Masters, but lost 5–10 to Judd Trump. At the 2024 World Championship, he defeated Mark Williams 10–9 in the first round and then lost 9–13 to Jak Jones in the last 16. The 2025 Masters marked his first appearance at the tournament, but Si lost in the first round to Mark Allen 2–6. At the 2025 World Championship, Si advanced to the quarter-finals where he was defeated by O'Sullivan 9–13.