Dewan was born in Hartford, Connecticut, the daughter of Nancy Smith (née Bursch) and Darryll Dewan, who was a running back on the 1972 Notre Dame football team. Her father is of Lebanese and Polish descent and her mother is of German and English ancestry. Her parents divorced when she was young and her mother remarried Claude Brooks Smith. During high school at Grapevine High School in Grapevine, Texas, Dewan-Tatum was a varsity cheerleader. She graduated in 1999 and was voted prom queen her senior year. She went to University of Southern California and was a member of the California Gamma Chapter of Pi Beta Phi.
In 2006, Dewan met actor Channing Tatum on the set of their film Step Up. They began dating shortly after filming had finished. The couple became engaged in early September 2008 in Maui, and married on July 11, 2009, at Church Estates Vineyards in Malibu, California. Their daughter, Everly, was born in May 2013 in London, England.
Dewan is an animal rights activist and a vegan. Dewan is raising her daughter as a vegetarian but says she will let her decide what to eat when she gets old enough.
Prior to her acting career, Dewan was a background dancer, working with singers such as Janet Jackson, 'N Sync, Sean Combs, Toni Braxton, Celine Dion, Pink, Missy Elliott, Ricky Martin, and Billy Crawford. Dewan appeared in Janet Jackson's music videos for "Doesn't Really Matter" and "All for You," and toured with Jackson on the All for You Tour, and later on 'N Sync's Popodyssey Tour. Dewan-Tatum appeared in a bit role as a backup dancer in the film The Hot Chick. She was also featured in Christina Aguilera's music video for the single "Not Myself Tonight" in 2010.Dewan first appeared in Janet Jackson's "Doesn't Really Matter" video, and later in "All for You" and the All for You Tour. Dewan credits working with Jackson for aiding her career, allowing her to later work with many other artists and giving her the experience necessary to co-star in the dance film Step Up.
In 2004, Dewan made her acting debut on the Fox sitcom television series Quintuplets. She has guest-appeared on The Young and the Restless, Joey and Melrose Place. In 2005, Dewan portrayed the protagonist in the independent medium-budget supernatural horror film Tamara. In 2006, she made appearances in The Grudge 2 and Take the Lead. That same year, Dewan starred as the female lead in the dance drama Step Up as Nora Clark, a dance student partnered with a troubled student who is sent to spend his community service as a janitor. The film made $119 million worldwide.
In August 2008, Dewan starred in the Lifetime made-for-television film Fab Five: The Texas Cheerleader Scandal which follows five rule-breaking teens. She portrayed the role of disciplinarian Coach Emma Carr. In November 2009, she appeared in the straight-to-DVD comedy American Virgin alongside Rob Schneider.
In August 2010, Dewan was cast in the ensemble crime heist film Setup. Production began in November 2010 in Grand Rapids, Michigan. The film was to be released theatrically in July 2011 but was released straight to DVD in September 2011.
In March 2011, Dewan was cast in the NBC pilot of The Playboy Club. In May 2011, NBC picked up the series for a full season. She portrayed the role of Bunny Janie, a provocative Playboy bunny who works at a Playboy Club in Chicago in 1961. The series premiered on September 19 to 5.2 million viewers with mixed reviews from critics. Due to low ratings, NBC cancelled the series after just three episodes in October 2011.
In November 2011, Dewan starred in the film The Jerk Theory. This was first released in Germany in 2009. It was later released in the US in November 2011.
Dewan appeared in the romantic-comedy film 10 Years, produced by her husband Channing Tatum. The film was released on September 14, 2012. That same year, she appeared in American Horror Story: Asylum, the second season of the horror television series, as Teresa Morrison. In 2013, Dewan began starring in the Lifetime series Witches of East End as Freya Beauchamp, one of the lead characters.
- Lucy Lane in Supergirl
- Freya Beauchamp in Witches of East End
- Janie in The Playboy Club
- Kendra Wilson in Melrose Place
- Welcome to Briarcliff - Teresa Morrison
- Tricks and Treats - Teresa Morrison
- Nor'easter - Teresa Morrison
- The Origins of Monstrosity - Teresa Morrison
- The Coat Hanger - Teresa Morrison