Tuomas Lauri Johannes Holopainen (born 25 December 1976) is a Finnish musician, songwriter, and record producer, best known as the primary songwriter, keyboardist, and founding member of symphonic metal band Nightwish. He has stated that his songwriting is influenced by harmonic film music.

Before founding Nightwish, he played in the band Darkwoods My Betrothed. He later recorded an album with them as a full member in 2020. He also collaborated with the gothic metal band For My Pain... and the band of Timo Rautiainen.

Holopainen has written several songs that have been included in movie soundtracks, including a collaboration with Nightwish bass player and male vocalist Marko Hietala on "While Your Lips Are Still Red", for the Finnish film Lieksa! in 2007. He also co-wrote the music for Nightwish's own film, Imaginaerum, released in November 2012.

Holopainen released his first solo album Music Inspired by the Life and Times of Scrooge in 2014.