On 24 May 2017, Tielemans joined 2016–17 Ligue 1 champions Monaco on a five-year deal for a fee of around €25 million. He made his competitive debut on 29 July in the Trophée des Champions at the Grand Stade de Tanger, playing the full 90 minutes in the 2–1 loss to Paris Saint-Germain and assisting Djibril Sidibé's opening goal. On 4 August, Tielemans made his Ligue 1 debut in the 3–2 home win over Toulouse, coming on as a substitute for Radamel Falcao in the 87th minute. He made his UEFA club competition debut for Monaco on 13 September in the UEFA Champions League group match away to RB Leipzig, playing the full 90 minutes and registering his first competitive goal for Monaco by equalizing in a 1–1 draw. On 16 September, Tielemans made his first Ligue 1 start and played the entire match in the 3–0 home win over Strasbourg, after having played a total of 56 minutes as a substitute in his first four Ligue 1 matches.