Wintersun is a Finnish heavy metal band from Helsinki originally formed as the side project of Jari Mäenpää, then-vocalist and guitarist of folk metal band Ensiferum.

In 2003, Mäenpää began pulling together songs that he had been working on since 1995. These songs followed specific themes described by Mäenpää as majestic, "spacy", and melodic. While Mäenpää planned to perform and record each instrument for the first album himself, he still required a drummer. He sent demos to Kai Hahto, former member of Rotten Sound, who agreed to perform the drums.

After obtaining a record deal, Mäenpää scheduled studio time to record the first album. Ensiferum's tour to support their 2004 release was scheduled at the same time, which forced Mäenpää to request time off from the band. The band instead fired him, and Mäenpää was free to go ahead with the production of his self-titled album, Wintersun.

After the release of Wintersun, the band found permanent members to perform shows and promote the album, although a keyboardist would never be found due to the increasingly-complex and expansive nature of the orchestrations and synth parts on the new music that Mäenpää was writing at the time. In 2006, Wintersun began work on their second and third albums, Time I and Time II, which were initially intended to be released as a single album. Recording began in May of that year, but the albums would be the recipient of years-long delays for a multitude of reasons, not the least of which was the complexity of each song's mix.

Two of the band's members, drummer Kai Hahto and bassist Jukka Koskinen, are also members of Finnish symphonic metal band Nightwish. In November 2023, Teemu Mäntysaari became a full member of American thrash metal band Megadeth, having filled-in on their most recent tour after Kiko Loureiro's departure.