Nicholas Ian Willis MNZM (born 25 April 1983) is a New Zealand middle-distance runner and the country's only two-time Olympic medalist in the 1500 metres. He won the silver medal at the 2008 Olympics in Beijing, and bronze at the 2016 Olympics in Rio De Janeiro. His other achievements over the same distance include the national and Oceania record (3:29.66), and medals from three consecutive Commonwealth Games (gold in 2006, bronze in 2010 and 2014).

Willis was born in and grew up in Lower Hutt, New Zealand, where he went to Hutt Valley High School and was coached by Don Dalgliesh before attending University of Michigan in the United States on a full athletic scholarship. His brother, Steve Willis, is also an athlete, and they are the only brothers in the history of New Zealand to have both broken 4:00 minutes for the mile.

In 2000, Willis won the 800m and 1500m double at the New Zealand Secondary Schools Championships. On the 20th of January, 2001, Willis became the fastest New Zealand secondary school student over the mile, with a time of 4 min 1.33 s.

In 2005, at the Golden League series in Paris, France, Willis broke John Walker's 32-year-old national record for the 1500m. Willis then won a Gold Medal at the 2006 Commonwealth Games, which he completed in 3:38.49 minutes, and two first placings at the New Zealand Track & Field Championships in 2006, with a time of 3:50.77 minutes, and 2008, which he ran in 3:44.46 minutes. His IAAF world ranking was 16 in May 2007.

Willis won the 1500m silver medal in the 2008 Summer Olympics. He originally placed third but the eventual disqualification of Rashid Ramzi due to a positive drug test saw Willis's bronze medal upgraded to silver, which he received in 2011. On 2 May 2018, it was announced that Asbel Kiprop of Kenya, who was declared the champion on Ramzi's disqualification, had tested positive for the banned performance-enhancing drug EPO Although this raises the possibility of Kiprop being disqualified and Willis being declared champion it is highly unusual for any drug sanction to cover more than a few years.

In the first 1500m heat Willis placed second in a time of 3:36.01. After running at the back of the field for the first few laps he took the lead on the final lap eventually finishing ahead of Mehdi Baala. Willis ran in the second 1500m semi-final where the pace was so slow that during the race he told the other competitors to "pick up the pace" so that more of them would have a chance to qualify. With 250m to go Willis was almost boxed in and had to force his way into the clear eventually finishing fifth in a time of 3:37.54 to qualify for the final. In the final Willis stayed at the back of the pack of runners for most of the race. Working his way through the field, he was in sixth place with 200m to go. Along the home straight he passed three runners, holding off Baala on the line to win the bronze medal in 3:34.16. With the disqualification of the race winner Rashid Ramzi on 18 November 2009 due to a positive drug test, Willis was upgraded to the silver medal.

In conclusion to his 2008 season, Willis won the Fifth Avenue Mile race in New York City, beating out twice-world champion Bernard Lagat of the United States by 0.1 seconds. Willis was the first New Zealander to win the race since John Walker in 1984.

During 2010, Willis underwent knee surgery. He came fifth in the Fifth Avenue Mile race and although he was the defending champion in the 1500m, his form leading into the 2010 Commonwealth Games was not his best. He managed to win the bronze medal in the event and said afterward that he was looking forward to training injury-free.