Matthew Arnold Thiessen (born August 12, 1980) is a Canadian-born American musician known for being the lead vocalist, pianist, guitarist, and primary songwriter for the rock band Relient K. With Relient K, he has released eight full-length albums, including three that were certified Gold, and three that peaked in the top twenty on the Billboard 200. Outside of his work with Relient K, Thiessen leads a side project called Matthew Thiessen and the Earthquakes, which released its debut album Wind Up Bird in 2018. In 2009, he co-produced and collaborated on Owl City's album Ocean Eyes.

Thiessen founded Relient K in 1998, with guitarist Matt Hoopes, and bassist Brian Pittman, and recorded the demo All Work & No Play the same year. The recording attracted the interest of Toby McKeehan, who signed the band to his label Gotee Records. Relient K was released on April 25, 2000, the group's first full-length CD. The band released their second studio album, The Anatomy of the Tongue in Cheek, on August 28, 2001, which was certified Gold by the Recording Industry Association of America. The band's third studio album Two Lefts Don't Make a Right...but Three Do was released on March 11, 2003, and was also certified Gold by the RIAA. On November 2, 2004, their fourth studio album Mmhmm was released on both Capitol Records and Gotee, and peaked at number fifteen on the Billboard 200. It was certified Gold by the RIAA in 2005. In 2007, Gotee, Capitol, and EMI released Relient K's fifth studio album, Five Score and Seven Years Ago, which peaked at number six on the Billboard 200. After the release of the double EP The Bird and the Bee Sides in 2008, the band signed with Gotee's Mono Vs. Stereo imprint, on which they released their sixth studio album, Forget and Not Slow Down, again reaching the top twenty of the Billboard 200. Thiessen and the other members of Relient K have taken up A&R positions for the label as part of the record deal.

Thiessen has been the lead vocalist, frontman, guitarist, primary songwriter, and, along with guitarist/backing vocalist Matt Hoopes, the only member who has been in the band for its entire history. Because of this position of musical leader, he maintains the most creative and artistic control of the band members. He has also served as co-producer on several occasions as well. As the band matured musically, Thiessen started being credited with piano in most releases, beginning with The Anatomy of the Tongue in Cheek in 2001. Since the release of Mmhmm in 2004, his piano playing started becoming more prominent, and he began including a piano onstage as well.

In addition to being in Relient K, Thiessen plays in his own solo piano project called Matthew Thiessen and the Earthquakes. Prior to 2018, the project had released a total of five songs on various samplers and compilation albums from Gotee and Mono Vs. Stereo, and several songs written for the project have been re-recorded on Relient K releases. In 2010, after the release and supporting tour of Forget and Not Slow Down, Thiessen announced that he would begin work on a full-length solo album. Although he had previously expressed interest in recording a studio release, he had placed his priority on Relient K, and relegated his solo work to his free time.

In April 2018, Adam Young of Owl City announced that Matthew Thiessen and the Earthquakes would be joining them on tour later in the year. In June 2018, Thiessen announced that his debut album under Matthew Thiessen and the Earthquakes, Wind Up Bird, would release later that summer.

Thiessen has occasionally collaborated with Jon Foreman of Switchfoot. After initial interest in recording vocals for a Switchfoot song, it instead worked out for Foreman to contribute vocals for the Relient K song "Deathbed" from Five Score and Seven Years Ago. He later contributed songwriting and vocals to the Switchfoot release "Rebuild" in 2007, in conjunction for the Appetite for Construction Tour.

In 2009, he contributed vocals, songwriting, and production to Owl City's major-label debut Ocean Eyes, which has sold over 4,000,000 copies worldwide and reached the top ten on the Billboard 200. Lead single "Fireflies" reached number one on the Billboard Hot 100 in late October, and was a global success, topping the charts in many countries including Australia, Denmark, the Netherlands, and the United Kingdom. Thiessen continues to work with Owl City's Adam Young and contributes vocals to "Plant Life", a song on the album, All Things Bright and Beautiful. He also co-wrote "Good Time" with Owl City and Brian Lee. Thiessen has also made guest contributions on a number of releases with other artists, including Blackbear, John Reuben, tobyMac and The Fold.

Thiessen has received piano lessons since he was seven years old, with early interests in ragtime and Scott Joplin, specifically the soundtrack from The Sting. He taught himself guitar at the age of 14 by learning to play songs by grunge bands like Silverchair and Nirvana. Thiessen does not use guitar picks while playing; he instead uses his thumb. According to Matt Hoopes, this is because he can't. Since Relient K's early days, Thiessen's main influences have been in the pop punk genre, specifically NOFX, Less Than Jake, Ghoti Hook, MxPx, and Goldfinger. He later introduced influences from Simon & Garfunkel, Ben Folds, and The Beach Boys, whom he has named as his favorite band. Thiessen has claimed that after years of writing and performing pop-punk songs, he now finds writing slower music more enjoyable. He has described his Matt Thiessen and the Earthquakes side project as being influenced by Rufus Wainwright and Ben Folds.