Gauthier Onclin (born 26 February 2001) is a Belgian tennis player who plays mainly on the ATP Challenger Tour.

[2][3] In January, Onclin reached the semifinals of the 2023 BW Open in Ottignies-Louvain-la-Neuve where he lost to eventual champion and compatriot David Goffin.

[7] In October, he received a wildcard in doubles for the European Open in Antwerp, alongside compatriot Michael Geerts.

[10][11] In October and November 2024 he reached two M25 finals in Portugal where he faced the same opponent, losing the first to Frederico Ferreira Silva but winning the second at the Vale do Lobo Open III.

[13] He started the season by increasing his 2024 winning streak to 13 matches[14] after he made it through the qualifications at the 2025 Australian Open into the main draw for his Grand Slam debut, defeating Roman Andres Burruchaga and Americans Aleksandar Kovacevic and Mackenzie McDonald.