Guillaume Couillard (born 10 December 1975) is a professional Monegasque former tennis player and a coach.

Couillard reached his career-high ATP Tour singles ranking of World No.

He is currently coaching Monegasque players Hugo Nys, Romain Arneodo and Lucas Catarina.

In 2013, Couillard and Benjamin Balleret played the longest known tiebreak in professional tennis history, lasting 70 points (34–36).

Couillard lost the match 6–7(34), 1–6 in the third qualifying round for the USA F1 Futures in Plantation, Florida.