Mark James McGuinness (born 5 January 2001) is a professional footballer who plays as a centre back for EFL Championship club Sheffield United. Born in England, McGuiness represents the Republic of Ireland national team.

Born in Slough, McGuinness joined Arsenal at the age of 10. After featuring regularly for Arsenal's under-18s side during the 2018–19 season, he signed his first professional contract with the club in April 2019. He broke into Arsenal's under-23 squad during the 2019–20 season, although he missed much of the season through injury. He featured for the Arsenal first-team for the first time during pre-season for the 2020–21 season, including scoring a header in a 4–1 win over Milton Keynes Dons in a friendly on 25 August.

McGuinness signed on a season long loan for Ipswich Town in September 2020. He made his senior debut on 27 October 2020, keeping a clean sheet in a 1–0 win over Gillingham. He scored his first senior career goal in a 1–0 away win over Burton Albion on 16 January 2021. McGuinness made 25 appearances during his loan spell at Ipswich, scoring once.

He signed for Cardiff City on 21 June 2021, on a three-year contract. He scored his first goal for Cardiff in a 2–1 win against Preston North End on 20 November 2021.

On 18 August 2022, McGuinness joined Sheffield Wednesday on a season-long loan. Two days later he made his debut, starting against Bolton Wanderers. He would win the clubs player of the month for November, making five appearances and scoring his first goal for the club against Shrewsbury Town. Another three clean sheets from a possible five in December saw him get back-to-back player of the month awards at the club. He was recalled back to Cardiff City on 19 January 2023 having played 23 times.

On 17 August 2023, it was announced that McGuinness had signed a new four-year deal that would keep him at Cardiff until the summer of 2027.

In August 2024, he transferred to Luton Town for a "significant" sum.

On 1 September 2025 he signed for Sheffield United.