Virtus Bologna (2012–2016) Fontecchio was promoted from the Virtus U19 to the senior side in 2012–13, alternating between the two during the season. He played a symbolic minute in LBA on 4 November 2011 before posting his first genuine first division stat sheet, missing his five shots for three rebounds and a steal, on 30 December. After an active youth career and a professional debut at only 17, there was talk of him going to the United States to play high school and later on
college basketball, however he stayed loyal to Virtus and signed a five-year professional contract with the team in June 2013, his parents had to be present as he was still a minor at the time. After twice being selected to take part in the league All Star Game (2014 and 2015), he was voted
Best Player Under 22 in May 2015. That season would also see his first participation in the
playoffs, scoring 10.3 points and grabbing 3.3 rebounds per game as Virtus were swept by title holders
Olimpia Milano. Fontecchio declared for the
2015 NBA draft on 25 April as an early entrant, two years before being automatically eligible in 2017. He attended a draft workout with the
Boston Celtics in June 2015, whilst they declaredly considered picking in the second round, he withdrew his candidacy on 16 June 2015.
Olimpia Milano (2016–2019) On 8 July 2016, Fontecchio signed with
Olimpia Milano of the
Lega Basket Serie A and the
EuroLeague. After an unsatisfying 2018–19 season with Olimpia, where Fontecchio played an average of less than a minute
Euroleague and 2.8 minutes in
Serie A, Fontecchio was set to join his former team
Virtus Bologna, coached by
Aleksandar Đorđević, but the agreement was not finalized.
Reggio Emilia (2019–2020) On 18 July 2019, Fontecchio signed with
Reggio Emilia in the Italian
Lega Basket Serie A (LBA).
Alba Berlin (2020–2021) On 7 July 2020, Fontecchio signed with
Alba Berlin of the
Basketball Bundesliga (BBL). It was here where Fontecchio broke out, averaging 10.6 points in over 23 minutes per game.
Baskonia (2021–2022) On 9 July 2021, Fontecchio signed a three-year deal with
Baskonia of the
Liga ACB.
Utah Jazz (2022–2024) On 17 July 2022, Fontecchio signed a two-year, $6.25 million contract with the
National Basketball Association's (NBA)
Utah Jazz. Fontecchio made his NBA debut on 19 October. On 7 December, Fontecchio made a game-winning dunk in a 124–123 win over the
Golden State Warriors. Fontecchio's career high came on 14 December 2023, against the
Portland Trail Blazers, where he scored 24 points on 87% field goal percentage. Fontecchio tied his career-high 24 points on 2 January 2024, in a 127–90 win against the
Dallas Mavericks.
Detroit Pistons (2024–2025) On February 8, 2024, Fontecchio was traded to the
Detroit Pistons in exchange for
Kevin Knox II, a second round pick and the rights to
Gabriele Procida. In May 2024, Fontecchio underwent surgery on his left big toe. On July 12, 2024, Fontecchio re-signed with the Pistons on a two-year, $16 million contract.
Miami Heat (2025–present) On July 7, 2025, Fontecchio was traded to the
Miami Heat in exchange for
Duncan Robinson. On March 10, 2026, Fontecchio's Heat teammate
Bam Adebayo scored 83 points, the second-most of all time in a single NBA game. With Fontecchio, the next leading scorer, putting up 18 points, the two became the second-highest scoring duo in a single NBA game, only behind
Wilt Chamberlain and
Al Attles in
Chamberlain's 100-point game in 1962. ==National team career==