Sels started his youth career at Kontich, but transferred to Lierse at the age of seven. Over there, he started in 2010 in the first team. After the departure of Eiji Kawashima to Standard Liègein the summer of 2012, Sels became the first choice keeper. In his first game in the First Division, on 29 July 2012 against Gent, he saved a penalty. At the start of the 2013–2014 season, when he refused to sign a new contract, he was demoted to the second team.
KAA Gent
On 31 December 2013, it was announced that Sels would complete a transfer to KAA Gent once the transfer window opened. He would become the number one goalkeeper as Frank Boeckx's contract was due to expire at the end of the season. Sels made his debut for AA Gent on 14 January 2014 against K.V. Kortrijk, in the quarter finals of the Belgian Cup. He helped AA Gent qualify for the next round after penalties; with the game ending 1–1 on aggregate. Sels played a key role in helping AA Gent become champions of the 2014–15 Belgian Pro League; the first national championship in their club history. This triumph allowed AA Gent to qualify for the 2015–16 UEFA Champions League. On his Champions League debut Sels dramatically saved an Alexandre Lacazette penalty to help his side secure a 1–1 draw with Olympique Lyonnais. On 13 January 2016 he was voted Best Goalkeeper of Belgium 2015.
On 29 June 2016, Sels completed a transfer to Newcastle United on a five-year contract for a reported transfer fee of £6.5 million. Sels made his Newcastle debut in the Championship on 5 August against Fulham. Sels kept a run of four clean sheets spanning 20 August to 13 September. Sels came under considerable fire from Newcastle fans for allowing a late equaliser from Aaron Tshibola at Aston Villa on 24 September. Despite receiving endorsement from manager Rafa Benítez during the week, Sels lost his place in the lineup to Karl Darlow against Norwich City on 28 September. His displacement from the lineup also received notable media coverage in his native Belgium. When Newcastle's next League Cup tilt came up, it was Sels who got the start, enjoying a clean sheet in a 6–0 victory over Preston North End on 25 October.
Sels was called up to the Belgium squad in October 2015 for Euro 2016 qualifiers against Andorra and Israel. He was named in the Roberto Martinez Belgian squad for the friendly against Saudi Arabia in Brussels on 27 March. In May 2018 he was named in Belgium’s preliminary squad for the 2018 World Cup in Russia. However, he did not make the final 23.
