Trossard joined the K.R.C. Genk youth academy from the Bocholt academy in 2010. He was promoted to the senior squad in 2012, making his first, albeit brief, debut on 13 May 2012 in a league match against K.A.A. Gent. The game ended 3-1 in favor of Gent. Trossard was introduced in the 87th minute, replacing Stef Peeters. Those three minutes were the only ones he played in the league that season. For several seasons, he was consistently sent out on loan. As a result, he didn't score his first league goal for Genk until over four years later, coming in a 4-1 away loss to K.V. Kortrijk on 25 September 2016. He was brought on as a substitute for Leon Bailey and scored in the 63rd minute, assisted by Bryan Heynen. In January 2013, Trossard was sent out on loan to Lommel United. In twelve league appearances, he scored seven goals. His debut for Lommel came on 3 February 2013 in a 1-0 away loss to Sint-Truiden. He was introduced as a substitute in the 63rd minute, coming on for Thomas Jutten. Arguably, the greatest game during his loan spell was his hat-trick in a 3-2 home victory over K.F.C. Dessel Sport, in front of 2000 fans at the Soevereinstadion. He scored one in the 11th minute, another in the 60th, and the last in the 74th. He was subbed off in the 88th minute. In July 2013, Trossard was loaned out to K.V.C. Westerlo. In 17 league appearances, he scored three goals. The first of these goals came on 9 August 2013 in a 2-0 away win over his former club Lommel United. He scored in the 74th minute. He made his debut on 3 August 2013 in a 2-2 home draw with A.F.C. Tubize. This was also his first start for the club. Trossard helped Westerlo to promotion to In July 2014, Trossard was loaned out to Lommel United again. He continued his goal scoring form, finding the net 16 times in 33 league matches. He scored his second career hat-trick for Lommel on 22 March 2015 in a 6-0 demolition of Racing Mechelen. His goals came in the 33rd, 61st, and 69th minutes. That season, Lommel were only nine points off promotion back to the top flight. In July 2015, Trossard was loaned out again, this time to OH Leuven. In 30 league matches, he scored eight goals. He made his Leuven debut on 25 July 2015 in a 3-1 away loss to the club he had been loaned out from in Genk. Genk didn't enforce the rule of not allowing loanees to play against them, as he was brought off the bench in the 61st minute, replacing Yohan Croizet. His first league goal for Leuven came on 16 August 2015 in a 2-0 victory over Charleroi. He made an immediate impact off the bench. He was brought on for Yohan Croizet in the 82nd minute, and scored in the 85th.
Brighton & Hove Albion
On 26 June 2019, Brighton agreed to sign Trossard, who joined on a four-year deal, becoming the club’s second signing of the summer, following Matt Clarke’s arrival from Portsmouth. Trossard made his debut for The Seagulls on 17 August where he scored the equaliser in the 1–1 home draw against West Ham United. Earlier in the game he was incorrectly denied a goal for offside by VAR. Trossard scored twice against Norwich in the 2019–20 season, one in the 2–0 home victory on 2 November 2019, and again in a crucial 1–0 away win on 4 July 2020 to help Brighton push further away from the bottom three.
International career
Trossard has been called up to the Belgium national football team on multiple occasions but has yet to earn his first cap. Roberto Martinez first named him in the Belgium squad in September 2018, again one month later when Trossard had to withdraw through injury and most recently Trossard remained on the bench again in March 2019.
