Debby Applegate is an American historian and biographer. She is the author of Madam: The Biography of Polly Adler, Icon of the Jazz Age and The Most Famous Man in America: The Biography of Henry Ward Beecher, for which she won the 2007 Pulitzer Prize for Biography or Autobiography.

Born in Eugene, Oregon, Applegate attended Amherst College as an undergraduate, where she began a two-decade fascination with famous alumnus Henry Ward Beecher, a 19th-century abolitionist minister who was later the subject of a widely publicized sex scandal. She made Beecher the subject of her dissertation in American Studies at Yale, where she received a Ph.D. After several more years of research, Applegate published The Most Famous Man in America, which was praised by critics and awarded the Pulitzer Prize. Her second book, Madam: The Biography of Polly Adler, Icon of the Jazz Age, an account of the life and times of the notorious Manhattan brothel-keeper Polly Adler, was published in November 2021 after thirteen years of extensive research.

Born in Eugene, Oregon, Applegate grew up in Clackamas, Oregon, graduating from Clackamas High School. She was raised in what she described as an "unusual religious environment": her mother, from a Mormon family, became a New Thought minister, while her father was an Irish Catholic. She graduated summa cum laude from Amherst College in 1989 and was a Sterling Fellow at Yale University, where she earned a Ph.D. in American Studies.

Applegate has taught at Yale, Wesleyan University, and Marymount Manhattan College. Her contributions have appeared in The Journal of American History and The New York Times.

Applegate was a founding member of Biographers International Organization (BIO), and served as its initial interim president in 2009. She currently serves as the Chair of BIO's Advisory Committee.

She is married to Bruce Tulgan, a business writer whose books include It's Okay To Be The Boss. They live in New Haven, Connecticut.

As an undergraduate student worker at Amherst College, Applegate was assigned to assemble an exhibit on a famous alumnus and selected Henry Ward Beecher, a 19th-century minister known for his abolitionist preaching and widely publicized sex scandal. Applegate described him as " unlike any religious figure I’d ever seen. I loved his very modern sense of humor, his irreverence, and his joyful, ecumenical approach to religion and life in general." She later made him the subject of her undergraduate senior thesis and her PhD dissertation at Yale University. After graduation, Applegate signed a publishing contract to write a biography of Beecher.

"I had acquired an excellent education as an academic historian, but I’d never had a single lesson, formal or informal, in this new craft I had so blithely chosen," Applegate later recalled. Applegate's initial chapters were written in what she considered an overly academic voice, so to write a biography with popular appeal, she studied fiction writing, including techniques for suspense and pornographic writing. "I made my way through my first book by trial and error, using my cobbled-together collection of examples, borrowed exercises and jerry-rigged postulations, to navigate the enormous task of fashioning an intellectually and emotionally compelling account out of the scattered detritus of a person’s life," wrote Applegate in a 2016 essay on making the leap from academic historian to popular biographer. She structured the resulting book as a psychological thriller.