[7][8][9] Courtois began his career with Genk and made his senior debut in 2009, at age 16; he went on to establish himself as the club's starting goalkeeper and won the Belgian Pro League.

Courtois has won the Ricardo Zamora Trophy three times, the Premier League Golden Glove once, and was named the third-best goalkeeper of the decade (2011–2020) by the International Federation of Football History & Statistics (IFFHS).

After turning down a transfer to TSG 1899 Hoffenheim of the Bundesliga, Courtois was made first-choice goalkeeper for the 2010–11 season ahead of Casteels, when Laszlo Koteles' registration ran into issues.

[13][14] In July 2011 Courtois joined Premier League club Chelsea for a reported €9 million, signing a five-year deal.

[25] Atlético reached the 2012 UEFA Europa League final, and Courtois kept a clean sheet as the club won 3–0 over fellow Spanish side Athletic Bilbao.

[27] His first game of the loan extension was against parent club Chelsea in the 2012 UEFA Super Cup in Monaco, which ended in a 4–1 victory for Atlético Madrid.

Later in the season Courtois set a new Atlético Madrid record of 820 minutes without conceding a goal at the Estadio Vicente Calderon,[28] ended by being beaten in a 1–0 loss to Real Sociedad.

[32][33] By conceding the fewest goals in a substantial number of matches during the 2013–14 season, Courtois won the Ricardo Zamora Trophy and made an important contribution to Atlético's 2013–14 La Liga title, their first since 1996.

[34] Atlético, however, lost the UEFA Champions League final 4–1 after extra time against neighbours Real Madrid on 24 May in Lisbon.

[37] On 18 August, Mourinho announced that Courtois would start the Premier League opener against Burnley in place of Petr Čech.

[38] Courtois kept his first Premier League clean sheet in his second game, making several crucial saves in a 2–0 home win over Leicester City.

"[40] Courtois suffered a head injury in the first half of Chelsea's 2–0 home win over Arsenal on 5 October due to a collision with Alexis Sánchez; he was substituted and then taken to hospital.

[46] Courtois suffered a leg injury in training on 11 September which required surgery and meant that he was expected to miss the next three months of competition.

[47] On 16 April, he was sent off at home to Manchester City for conceding a penalty with a foul on Fernandinho; Sergio Agüero converted it past Begović to seal a 3–0 win.

[53] In April 2017, Courtois was ruled out for the match against rivals Manchester United, as a result of suffering a reported ankle injury, which he sustained in training.

[59][60] After a strong performance at the 2018 FIFA World Cup, where he won the Golden Glove for best goalkeeper, Courtois expressed his interest in moving on.

[69] In the shootout, Saul saw his spot-kick hit the post before Thomas' effort was saved by Courtois, giving Ramos the chance to secure Real's 11th Spanish Super Cup triumph.

On 28 May 2022, Courtois was named player of the match in the 2022 UEFA Champions League final,[76] where he managed to make a total of nine saves,[77] which helped Real Madrid to beat Liverpool 1–0.

[78] On 10 August 2023, Real Madrid announced that Courtois was diagnosed with a torn anterior cruciate ligament in his left knee.

[94] Courtois then managed to keep consecutive clean sheets in 1–0 wins against Russia and South Korea as the Red Devils reached the quarter-finals, where Argentina eliminated Belgium with Gonzalo Higuaín scoring the only goal of the match in the eighth minute.

Afterwards, Courtois hinted that Belgium coach Marc Wilmots was at fault for the loss and also stated that the defeat was the "biggest disappointment" of his career.

[106] On 22 August 2024, Courtois announced that he would not return to the national team for the time being, citing the conflict with Tedesco as the main reason.

[108] A consistent keeper,[108] he possesses an excellent positional sense,[110][111] good mentality, composure, strength of character,[110][112][113] and an ability to communicate well with his defenders;[108] he also excels in one on one situations, due to his ability to time his runs effectively when rushing out of goal to face opponents,[108][111] and has even functioned as a sweeper-keeper on occasion.

[10] Thibaut's older sister Valérie Courtois is a volleyball player who plays as a libero for Stade Français Paris Saint Cloud and Belgium.

[129] Inspired by Courtois' pose from a particular save in January 2013, a Colombian fan created a new widespread social media meme called 'Thibauting' to pay homage to the Belgian goalkeeper.

In February 2014, Courtois caused some stir in the national team, by saying of Simon Mignolet, his rival for the starting goalkeeper spot, "You have to know how to stay humble and respectful, and he should remember that.

[134] In April 2018, Wilmots, by then out of a job, accused Courtois and his father Thierry of leaking Belgium's line-ups before they should have been revealed to the public.

Courtois holds an administrative role within the team and hired Roberto Merhi as sporting director and driver coach.