Jeffrey Wayne Fatt AM (born 21 July 1953) is an Australian musician and actor. He was a member of the children's group The Wiggles from its founding in 1991 to 2012, and was also in the 1980s and 1990s pop band The Cockroaches with fellow Wiggle Anthony Field.

He was the oldest member of the original Wiggles line up. Fatt was also known for falling asleep on stage, causing the other members (and later on the audience) to utter the iconic Wiggles catchphrase "Wake Up, Jeff!", which was also the title of one of their songs, as well as the title of one of their videos and albums.

The second youngest of four, Fatt's Chinese parents Norma Wong Yee and Oscar Fatt owned a large retail store in Casino, New South Wales. His great-grandfather immigrated to Sydney, Australia in 1896, becoming a prosperous banana businessman. Fatt had aspirations to become an architect, and earned a Bachelor of Arts in industrial design. Anthony Field described him as a "bush kid" who "first learned to let his keyboard do most of the talking".

He opened a public address equipment business with his brother in Sydney when he began playing keyboard for the rockabilly band the Roadmasters, and was recruited by Anthony and Paul Field to join the Cockroaches. Fatt joined the Cockroaches in 1981, serving as their keyboardist. In 1986 they signed with an independent label, Regular Records, which issued their first three albums, including The Cockroaches (March 1987), which peaked at No. 9 on the Australian Kent Music Report Albums Chart; it sold 70,000 copies and was certified platinum by their label. The album spawned the single "She's the One", which became the band's biggest hit when it peaked at No. 7 on the Kent Music Report Singles Chart in April 1987.

After The Cockroaches disbanded in the early 1990s, Anthony Field asked Fatt, who was renovating his house at the time, to help them record The Wiggles' first album in 1991. Fatt reported, "I got this call from Anthony: 'Hello, Anthony. Go away, go away. I'm not interested', you know. He's always got these ideas for things. He said, 'I've got this idea, come down to the studio'". Fatt's response to Field's request was, "Sure but how long will it take?" Field reported that as Fatt had done for the Cockroaches, he was "a musical backbone for the Wiggles". As a Wiggle, Fatt wore a purple skivvy.

He originated the Wiggles character Henry the Octopus and has occasionally voiced Wags the Dog, and performed those voices when other actors took over the roles. Fatt reported that it took twelve months to "learn the language of preschool", including addressing children using the inclusive "everybody", instead of "boys and girls", and that the Wiggles' first performances were traumatic for him because he was not used to being around children.

Fatt's schtick was sleeping at odd times, which led to a Wiggles catchphrase and the title of one of their songs, "Wake Up, Jeff!" Fatt was the oldest Wiggle and was well known for his "laid-back personality", which according to Field, made him "invaluable company on the road". As Sam Moran has said, when referring to the development of the Wiggles' shticks, "Jeff really does fall asleep", and Fatt reported, "The irony is I'm a very light sleeper in the nighttime. I will invariably have a snooze before the show". Fatt was the only member of the Wiggles without a background in early childhood education; he stated that was the reason falling asleep was chosen as his schtick, "because it was a way of getting me involved in the shows without actually having to do anything". Field claimed Fatt's schtick endeared Fatt to their audiences and called it "a simple audience participation and interaction gag we've done since the start of the group".

Kathleen Warren, the group's former professor at Macquarie University and their consultant for their shows and DVDs, believed that the shtick empowered children, and Paul Field reported that children in the Wiggles' audience felt "great excitement" and were disappointed if not given the opportunity to help Jeff in this way.