This is a list of seasons of the National Hockey League (NHL), a professional ice hockey league, since its inception in 1917. The list also includes the seasons of the National Hockey Association (NHA), the predecessor organization of the NHL, which had several teams that would continue play in the NHL.
Only two franchises, Montreal and Toronto, still exist from the founding of the league. The Quebec Bulldogs, which suspended after the last NHA season, returned to play in the third NHL season, although they were considered founding members of the NHL. The team would be moved by the league to Hamilton, and eventually dissolved by the league in 1925. The original Ottawa Senators would continue in the league until 1935, where, after one season in St. Louis as the St. Louis Eagles, the franchise was dissolved by the league. The current Ottawa Senators franchise does recognize the history of the original Senators (through retired numbers and a heritage jersey).
The list is sub-divided using the same eras as the series of articles on the History of the National Hockey League.
Like predecessor leagues, the champion of the NHA league since its founding was the team with the best regular season record, with a playoff only used if more than one team had the best win-loss record. This changed in 1917 with the invention of the split-season, whereby the champion became the winner of the annual playoff. The NHL continued the split-season and playoff format upon the winding up of the NHA organization. Except for the 1919–20 season, where there was no playoff because Ottawa won both halves of the season, the champion of the NHL has been the playoff champion.
The NHA champion was awarded the O'Brien Cup. This was continued by the NHL. Until 1927, the NHL champion was awarded the O'Brien Cup, supplemented by the Prince of Wales Trophy, starting in 1925. To win the Stanley Cup, the NHL champion had to play off in a "world's series" with the champion of the Pacific Coast or Western hockey leagues. After 1927, the NHL playoff champion was awarded the Stanley Cup, while the O'Brien Cup and Prince of Wales Trophy were reused as division championship and playoff runner-up awards.