Nathaniel Joseph Ruess (pronounced ROOSS; born February 26, 1982) is an American singer and songwriter. He formed the indie pop 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.