Judith Rumelt, better known by her pen name Cassandra Clare, is an American author of young adult fiction, most known for her bestselling series The Mortal Instruments.

Clare was born Judith Rumelt, to American parents, in Tehran (Iran). Her parents are Elizabeth and Richard Rumelt, who are a business school professor and author. Her maternal grandfather was film producer Max Rosenberg. Clare is Jewish, and has described her family as "not religious".

As a child, Clare travelled frequently, spending time in Switzerland, England, and France. She returned to Los Angeles for high school, and from then on split her time between California and New York, where she worked at various entertainment magazines and tabloids, including The Hollywood Reporter.

While living in Los Angeles Clare began writing fan fiction using the name Cassandra Claire. The Draco Trilogy based on Harry Potter and The Very Secret Diaries based on The Lord of the Rings were popular. However, she deleted her fan fiction from the Internet shortly before her first novel, The City of Bones, was published under the name Cassandra Clare.

She is also friends with author Holly Black, and their books occasionally overlap, Clare mentioning characters from Black's novels and vice versa, such as Val and Luis from Black's Valiant.

Clare is also credited by her publisher with creating the "City of Fallen Angels treatment" where a tangible "letter" from one character to another is attached to the back of physical copies of a book. The goal is to spur print book sales.

She currently resides in Amherst, Massachusetts, with her husband, Joshua Lewis, and three cats.