Early career Denayer started playing football at age six, where he started out at FC Ganshoren and then
Anderlecht. At the age of 13, he joined the
JMG Academy in
Lier. It was there he was converted from playing as a striker to becoming a defender. Denayer credits his time at JMG for honing his football technique. Youths at the academy do not play normal competition football, instead taking part in small-sided games among each other, focusing on technique and education in a private school. They trained in bare feet to improve their touch; "It helps to develop your technical skills. Once you've reached a certain level, you're allowed to put on shoes. From then on the training is focused on technique and playing in small spaces" explained Denayer. He described the regime as "tough", adding "Your day started at 7 am and ended at 10:30 pm. In between you had 90 minutes – just before bedtime – to call your friends or family. It was playing football, studying, eat, sleep". In 2013,
Lierse, who are in partnership with the JMG Academy, attempted to sign Denayer. However, he rejected their offer and instead moved abroad to England to try and find a club. He had a trial at
Liverpool, but club and player failed to reach an agreement. He then went for a trial at
Manchester City. Denayer was ever present for the club's campaign in the
UEFA Youth League, where he made eight appearances, scoring once in a 6–0 win over
Bayern Munich. Denayer also played a huge role in domestic competitions where he helped the club finish fourth in the
Professional U21 Development League, as well as reaching the final of the
U21 Premier League Cup. In pre-season during July 2014, Denayer was included in the first team and started in a 2–0 friendly loss away against
Dundee. On 10 July 2015, it was confirmed that he would be in the club's squad for their pre-season tour in Australia. Denayer signed a new five-year deal on 13 July 2015.
Loan to Celtic On 12 August 2014,
Celtic signed Denayer on a season-long loan from
Manchester City, making him one of
Ronny Deila's first major signings as Celtic manager. Denayer revealed that
Vincent Kompany and
Patrick Vieira played a vital role for him in joining the club. He scored on his debut and was named
man of the match in a 6–1 victory at home to
Dundee United in the
Scottish Premiership. Four days later, on 20 August 2014, Denayer made his Champions League debut, in a 1–1 draw against
Maribor, where he was part of the starting line-up. His second goal came in a 2–1 win over
Aberdeen on 13 September 2014. The defender scored again for Celtic on 18 October 2014, in a 5–0 win over
Ross County. By the end of December, Denayer had made 17 appearances for Celtic and formed a solid partnership with
Virgil van Dijk in central defence. Denayer commented about playing alongside Van Dijk: "We play well together and he is a very good player", adding "I am also learning from him as well. He has more experience than me and he knows what to say to me. I listen to him because I know he can help make me a better player." However, Denayer immediately ruled out joining Celtic on a permanent basis, insisting that he hoped to earn a first team place at Manchester City in the near future. Manchester City kept an eye on Denayer's progress at Celtic, with
Development Side coach Patrick Vieira stating "When I watch Jason playing for Celtic, or
Marcos Lopes playing at Lille, it makes us really proud. The way they are performing lets me know we are doing things the right stuff ", and adding "The object is to provide players for the first team, but we know the gap is massive from under-21 to the first team – but this is one way we can get them there." Denayer won his first major trophy on 15 March 2015 when he played in Celtic's 2–0 win over Dundee United in the
League Cup Final at
Hampden Park. He was described as being "strong in the tackle" and "comfortable with the ball". Denayer himself stated that "It is the first cup final and first win of my career, so it was a very special day for me." The next day, Denayer was voted the
PFA Scotland Young Player of the Year, with his teammate
Stefan Johansen taking
the overall prize. After winning the prize, he said when asked if he would remain at Celtic: "I don't know. I need to wait until summer and see what is going to happen... When I came here I just wanted to play some games and take some experience, but now I win trophies and personal trophies so it has gone better than I expected".
Loan to Galatasaray In August 2015,
Galatasaray signed Denayer on a season-long loan from Manchester City. Denayer played his first game for Galatasaray on 15 September 2015, in a
UEFA Champions League tie against
Atlético Madrid. Despite a 2–0 defeat, the Belgium defender turned in an impressive performance. He then made his league debut on 19 September in match away against
Trabzonspor, playing on the right side of defence, helping his new side keep a clean sheet in a 1–0 victory. Denayer continued to display his natural attributes of pace and power, whilst also improving his reading of the game. Two assists for goals against
Eskişehirspor and
Gaziantepspor also highlighted his ability to contribute to the attacking side of the game. His last match for Galatasaray was in the Turkish Cup Final against
Fenerbahçe on 26 May 2016. Denayer provided the assist for the only goal of the game on 30 minutes; a flick on from
Emre Colak's corner on to
Lukas Podolski who headed in from close range.
Loan to Sunderland On 31 August 2016, Denayer joined Premier League side
Sunderland on a season-long loan deal. However, Sunderland were poor and ended up being relegated. Denayer struggled to perform consistently and was often fielded in midfield instead of his natural position as a defender. He was also hindered by a series of niggling injuries towards the end of the season.
Second loan to Galatasaray On 31 August 2017, Denayer re-signed for
Galatasaray on a season-long loan deal. In a 2–0 win over
Osmanlıspor on 28 January 2018, Denayer completed 60 successful passes, overtaking the previous pass completion record in Süper Lig set by
Atiba Hutchinson of
Beşiktaş in season 2014–15. He played 22 games during his second spell at the club to help
Galatasaray win the
2017–18 Süper Lig.
Lyon On 21 August 2018, Denayer joined
Ligue 1 club
Lyon for €6.5m, signing a four-year contract. Denayer was a regular for the team, having 31 starts in the
2018–19 season and scoring two goals. He faced his former employers, Manchester City, in the
Champions League quarter-finals on 15 August 2020, as Lyon won 3–1.
Shabab Al Ahli After his contract with Lyon expired on July 1, Denayer was a free agent. He was signed by Emirati club
Shabab Al Ahli in late September 2022.
Al Fateh On 23 July 2023, Denayer joined
Saudi Professional League club
Al Fateh on a two-year deal.{{cite news|url=http://www.fatehclub.com/blog/lkhbr-5/lbljyky-jyswn-dynyr-fthwyan-54#blog_content|title=البلجيكي "جايسون ديناير" فتحاويًا On 31 January 2025, Denayer left
Al Fateh on mutual agreement and is currently a free agent. ==International career==