Nathaniel Joseph Ruess (pronounced /ruːs/ ROOSS; born February 26, 1982) is an American singer and songwriter. He formed the indie rock band The Format in 2002, and later went on to form the band fun. in 2008.

He emerged as a solo act with his guest performance on P!nk's 2013 single "Just Give Me a Reason", which earned two nominations at the 56th Annual Grammy Awards and won the MTV Video Music Award for Best Collaboration. His 2015 single, "Nothing Without Love", marked his first entry on the Billboard Hot 100 as a solo artist. It served as the lead single for his debut solo album, Grand Romantic (2015), which peaked at number seven on the Billboard 200 and saw positive critical response.

Ruess has also co-written a number of commercially successful singles, including "Die Young" by Kesha, "Stay the Night" by Zedd and Hayley Williams, "Walk Me Home" by Pink and with Keith Urban, Maroon 5 and Ellie Goulding. His songwriting work has earned him the Hal David Starlight Award at the 2015 Songwriters Hall of Fame.

Ruess was born the second child of Larry Ruess and Bess Zinger. His uncle, John Ruess, was a performer on Broadway and served as an influence for Ruess's musical ventures.

In 1986, his family moved to a farm in Glendale, Arizona, due to repeated bouts of pneumonia Ruess faced as a child. Ruess's pneumonia and the move are referenced in the lyrics of the song "The Gambler". Ruess remained in Arizona throughout his childhood and attended Deer Valley High School, graduating in 2000. During his time in school, Ruess played in punk bands and upon graduation chose to pursue music professionally. In an interview with American Songwriter Ruess states, "I'm not one to take lessons, so I decided that the only way I was going to learn how to sing, if what they were saying was true, was to go in my car and put on any sort of music from a vocalist that might be really hard to mimic, turn it on as loud as possible and try to hit all those notes". He took a job in a law firm as a way to support himself while he pursued his musical ambitions.

In 2002, at the age of 19, he launched the band the Format with his longtime best friend, Sam Means. The Format was his first musical endeavor to gain widespread attention.

The Format debuted in 2002 with a five-song EP, titled EP, which generated local interest and led to the band being signed to Elektra Records the same year. They released their first studio album, Interventions + Lullabies, on October 21, 2003, leading to more local mainstream success. The band's fanbase began to grow, and the Format released their second EP, Snails, with Atlantic Records in April 2005. However, while working on their second album, Dog Problems, they were dropped from Atlantic. They eventually created their own label, The Vanity Label, and released the album on July 10, 2006. On February 4, 2008, Ruess announced through the band's blog that the Format would not be making another album.

Immediately after the split of the Format, Ruess contacted Jack Antonoff of Steel Train and Andrew Dost, formerly of Anathallo, to form a new band called fun. They released their first demo, "Benson Hedges", through Spin's September 20, 2009, article. Four months after releasing their first single, "At Least I'm Not as Sad (As I Used to Be)", through Myspace on April 6, 2009, Fun released their first studio album, entitled Aim and Ignite. The album received generally positive reviews and peaked at 71 on the Billboard 200.