Frank Xavier Leyendecker, also known as Frank James Leyendecker, was an American illustrator. He worked with his brother Joseph Christian Leyendecker, in their studio, first in Chicago, then later in New York City and New Rochelle, New York.

He was born in Germany on January 19, 1876, as Franz Xavier Leyendecker.
Frank Leyendecker and his sister Augusta lived for a period of time (though 1924) with their brother J. C. Leyendecker and the model Charles Beach in New Rochelle.

He studied for a time at the Académie Julian in France. He was known for his stained glass work as well as his illustrations for posters, magazines and advertisements. He also painted covers for Street & Smith pulp magazines, such as People's Favorite Magazine and The Popular Magazine, as well as for Fawcett's pulp magazine Battle Stories. His painting for Battle Stories was originally created as a WWI recruitment poster that Fawcett Publications posthumously reprinted as a pulp magazine cover in 1931. Leyendecker served as the judge in the first Strathmore Water Color Contest, sponsored by the Mittineague Paper Company of Massachusetts. His work was described as an "important feature" of the second exhibition of the Society of Illustrators at the International gallery in New York.

Leyendecker was suffering from depression and poor health due to his ongoing drug addiction, when he most likely committed suicide by morphine overdose on April 18, 1924, at the age of 48.