Susan Jane Powter (born December 22, 1957) is an Australian-born American motivational speaker, nutritionist, personal trainer, and author who rose to fame in the 1990s with her catchphrase "Stop the Insanity!", the centerpiece of her highly successful weight-loss infomercial that generated over $50 million in annual revenue at its peak.

Born in Sydney, Australia, she immigrated to the United States at age 10, left school in ninth grade, earned a GED, and relocated to Dallas, Texas, in 1980, where she married Nic Villarreal in 1982 and had two sons before their 1986 separation amid his infidelity, prompting her weight to balloon from 130 to 260 pounds.

Susan Powter transformed herself by walking, eliminating sugar and processed foods, slimming to 114 pounds, and worked as a topless dancer before using an inheritance from her mother's 1988 death to open the Susan Powter Wellness Center in Dallas.​

Her infomercial launched a media empire, including three New York Times bestsellers, Stop the Insanity! (1993), The Pocket Powter (1994), and Food (1995), and a syndicated daytime talk show, The Susan Powter Show, produced by Woody Fraser and carried by nearly 200 stations for one season in 1994-1995, where she consulted on nutrition, fitness, and broader topics.

Susan Powter appeared as a health consultant on Home, guest-starred on Space Ghost Coast to Coast, and turned down a role in Kevin Costner's 1995 film Waterworld, while her barefoot, platinum buzzcut persona earned comparisons to Richard Simmons and Betty Friedan.

In 2002, she self-published The Politics of Stupid, critiquing food industries, governments, and diet fads akin to Fast Food Nation, evolving her motto from "Eat, breathe, and move" to include "think."​

Despite personal challenges including multiple marriages, three sons, legal battles, divorce, and bankruptcy that left her net worth at $50,000 by some estimates and living in an RV while doing food delivery by 2018, she released her memoir And Then Em Died... Stop the Insanity! A Memoir in October 2024. A 2025 documentary, Stop the Insanity: Finding Susan Powter, co-produced by Jamie Lee Curtis and directed by Zeberiah Newman, chronicles her life and resurgence.