J.K. Rowling’s full first name is Joanne.
She was born Joanne Rowling on 31 July 1965 in Yate, Gloucestershire, England. The author name “J.K. Rowling” does not represent two given names: the “K” was added for publishing purposes and stands for Kathleen, her paternal grandmother’s name.
Rowling has explained that her publishers believed a name containing initials might appeal more broadly to young male readers. She therefore used “J.K. Rowling” when Harry Potter and the Philosopher’s Stone was published in 1997. Her legal name later became Joanne Murray after her first marriage, and she has also published fiction under the name Robert Galbraith.
A common mix-up is assuming that “K” was part of her birth name. It was not. Rowling’s birth name was Joanne Rowling; Kathleen was selected as the second initial for her professional author identity.