Stefan Nillessen (born 2 January 2003) is a Dutch middle-distance runner.

[2] He won gold in the 1500 metres at the 2023 European Athletics U23 Championships in Espoo.

[6] He competed at the 2024 Summer Olympics over 1500 metres and ran a personal best to qualify for the final.

[7] Competing in Karlsruhe in February 2025 he won over 3000m in a Dutch indoor record time of 7:37.10.

Stefan Nillessen comes from a famous running family from Groesbeek, known as "d’n Boes."