Mathias Christiansen (born 20 February 1994) is a Danish badminton player who joined the national team in July 2013. He was the silver medalists in the 2018 and 2024 European Championships, and also the bronze medalists in the 2016, 2017, and 2021 European Championships, as well at the 2023 European Games. He was part of Danish winning team at the 2016 Thomas Cup. He competed at the 2020 Summer Olympics.

Christiansen was born on 20 February 1994, in Bornholm, Denmark, where his journey into badminton began at a young age in the town of Allinge. His introduction to the sport was quite serendipitous; he started playing after his schoolteacher encouraged him to attend badminton practice. Christiansen's talent quickly became evident, leading him to compete at the international level as a teenager, including a notable debut at the 2011 Scottish International. His early career reached a significant milestone in 2013 when he moved to Copenhagen to join the Danish national team, marking his transition into the professional circuit where he would eventually become a key figure in both men's and mixed doubles.

In 2017, Christiansen and Daugaard secured a bronze medal in the men's doubles at the European Championships and reached a career-high ranking of No. 19 in that category. However, his profile rose significantly in late 2017 when he paired with the legendary Christinna Pedersen. This duo quickly became a global powerhouse, reaching back-to-back finals at the China and Hong Kong Open. In 2018, the pair captured the India Open title and earned a silver medal at the European Championships in Huelva, Spain. Their consistent performance at elite tournaments, including a runner-up finish at the Korea Open, propelled Christiansen to a career-best world ranking of No. 4 in October 2018. Additionally, he helped the national team secure a bronze medal at the 2018 Thomas Cup in Bangkok.

Following Pedersen's retirement from international play in early 2019, Christiansen began his current partnership with Alexandra Bøje. Making their debut at the Canada Open in July, they reached the podium finishing as runners-up in the Hungarian International in November, and then claimed two titles in the Irish and Scottish Open.

Christiansen helping Denmark secure gold at the 2021 European Mixed Team Championships and contributing to back-to-back bronze medals at the 2020 and 2022 Thomas Cup finals. On the individual circuit, Christiansen and Bøje won the bronze medal at the 2021 European Championships. They also captured titles at the 2020 SaarLorLux Open and the 2021 Orléans Masters, reached the finals of the 2021 Swiss and French Open, and made his Olympic debut at the Tokyo 2020 Games. His consistency against the world's best culminated in a career-high world ranking of No. 8 in October 2022.

In 2023, Christiansen and Bøje were in top form, capturing titles at the Spain Masters and the Singapore Open, and securing a bronze medal at the European Games in Poland. This momentum continued into early 2024 as they reached the final of the European Championships in April, earning a silver medal, and were runners-up at the Canada Open in July.

However, just days before the 2024 Paris Olympics, for which they had qualified, Christiansen announced his withdrawal from the Games. The decision followed a series of three unintentional administrative errors in reporting his "whereabouts" to Anti-Doping Denmark over a 12-month period. To avoid disrupting the Danish Olympic team with the pending investigation, Christiansen chose to step down, which eventually resulted in a one-year suspension that lasted until July 2025.

The year 2025 marked a triumphant return to the professional circuit for Christiansen after completing a one-year suspension related to anti-doping whereabouts reporting. Reunited with his longtime partner, Alexandra Bøje, the pair made an immediate impact by winning the Macau Open in their very first tournament back in July. They carried this momentum into the BWF World Championships in Paris, where they pulled off a massive upset against the world No. 1 seeds, Feng Yanzhe and Huang Dongping. Throughout the fall, Christiansen and Bøje continued their high-level play, securing another title at the Hylo Open, and an additional victory at the Indonesia Masters Super 100. By the end of 2025, Christiansen had successfully rehabilitated his world ranking from outside the top 150 back toward the elite tier.