Phyllis Eisenstein (February 26, 1946 – December 7, 2020) was an American author of science fiction and fantasy short stories as well as novels. Her work was nominated for both the Hugo Award and Nebula Award.

Eisenstein was born Phyllis Leah Kleinstein in 1946 in Chicago, Illinois, and lived there most of her life. While studying psychology at the University of Chicago in 1963, she met her future husband Alex at a weekly gathering of Chicago's science fiction fandom. In 1966, shortly after attending Tricon, the 24th World Science Fiction Convention, they were married. She continued college until Alex entered the U. S. Air Force and, following basic training, was posted to Germany; they lived there for three years and then returned to Chicago upon his honorable discharge from the service.

Eisenstein had her first two science fiction stories published in 1971, the first in collaboration with husband Alex (he continued to be her writing partner for certain short stories). After establishing herself as a professional writer, she returned to college to finish her education, studying at the University of Illinois, where she earned a 1981 B.A. degree in anthropology.

She wrote eight novels, subsequently publishing six of them and nearly fifty shorter works of varying lengths in the genres of science fiction, fantasy, and horror fiction; Eisenstein also wrote a popular non-fiction book on the treatment of arthritis. Her stories have appeared in a number anthologies and in many major science fiction and fantasy print media magazines; these include The Magazine of Fantasy and Science Fiction, Analog Science Fiction and Fact, Galaxy Science Fiction, Isaac Asimov's Science Fiction Magazine, Amazing Stories, Weird Tales, and others.

Eisenstein's stories were nominated twice for science fiction's Hugo Award and three times for SFWA's Nebula Award.

Her 1978 short story "Lost and Found" was adapted for television in 1986, airing on the mid-1980s series The Twilight Zone (the first of three revivals of the classic series); the screenplay was written by the show's then story editor George R. R. Martin. She was an old friend of Martin and later convinced him to include dragons in his international best-selling fantasy series A Song of Ice and Fire. Martin then dedicated the third novel in the series, A Storm of Swords, to Eisenstein.

Eisenstein spent much of her adult life teaching writing; this began by assisting author Roger Zelazny at the Indiana University Writers Conference in 1977. She taught writing at the Clarion Science Fiction Writer's Workshop at Michigan State University, Oakton Community College of Skokie, Illinois, and the Writer's Digest School. For twenty years she was a member of the part-time faculty of Columbia College Chicago, teaching courses in general science fiction, popular fiction writing, fantasy, and advanced science fiction writing. In 1999, Eisenstein received an "Excellence in Teaching" Award from this institution; she retired from CCC in 2009 to devote more time to her professional writing career.

Beginning in 2000, Eisenstein began working full-time in Chicago's very competitive advertising business; she went on to become the executive manager of copy editors for more than a decade at Leo Burnett, Chicago's largest advertising agency, until declining health forced her retirement in 2015.