Stephen Welsh

c during January 2020. He made his first team debut for Celtic on 2 February 2020, in a 4–1 win away to Hamilton Academical.

On 6 February 2021, Welsh scored the first senior goal of his career with a header from a David Turnbull corner in the opening minutes of a 2–1 league win against Motherwell. On 13 April 2021, he signed a new contract with Celtic that is due to run until 2025. On 9 December 2021, Welsh scored an early header against La Liga side Real Betis in a UEFA Europa League match where Celtic won 3–2 at Celtic Park in their last group-stage game in the competition that season.

On 31 July 2022, Welsh scored the first goal in Celtic's 2022–23 Scottish Premiership opener against Aberdeen in a 2–0 home win. On 10 January 2022, it was reported that Serie A side Udinese had made a loan offer with an option to purchase attached for Welsh, which Celtic chose to reject.

In the summer transfer window, Ligue 1 side Toulouse made an approach for Welsh. However, Celtic remained uninterested in selling the player as manager Ange Postecoglou saw him as being part of first-team plans.

On 18 August 2023, Welsh signed a new four-year contract with Celtic, keeping him at the club until 2027.

Welsh has represented Scotland at under-17, under-19 and under-21 levels. He captained the under-21 side on numerous occasions. On 10 June 2022, he played his final game for the under-21s against Denmark in a 1–1 UEFA European Under-21 Championship qualifying draw where he faced Celtic teammate Matt O'Riley.