The name Gareth originated from the Welsh language and culture. It derives from the Germanic elements "gar" meaning "spear" and "rith" meaning "assault" or "prompt." The earliest known spelling was "Gararth" in the 9th century.

In Welsh mythology, Gareth was one of the Knights of the Round Table in Arthurian legend. He appeared as a character in the 14th century French chivalric romance Lancelot-Grail and the 15th century English Le Morte d'Arthur by Thomas Malory. Sir Gareth was known as the Beaumains or "Fair Hand" and the youngest son of King Lot and Morgause.

An early recorded bearer of the name was Gareth Ddu, a 14th century Welsh poet and son of Prince Llywelyn ap Gruffudd. Another was Gareth ap Bleddyn, a 12th century Welsh landowner and nobleman in Powys. Jeuan Gethin, born around 1392, was a Welsh language poet and court poet who used the bardic name Gareth Gethin.

In the 16th century, Gareth Harrison was an English explorer and privateer who sailed with Sir Walter Raleigh's expedition to Roanoke Island in 1585. Gareth Jones (1905-1935) was a Welsh journalist who exposed the Soviet famine genocide in Ukraine and was murdered under mysterious circumstances.

Gareth Edwards (1835-1915) was a Welsh minister, academic and writer who founded the Calvinistic Methodists and published writings in the Welsh language. Sir Gareth Rhys (1916-1992) was a Welsh international rugby player and Royal Navy officer during World War II.