Harry Kellar (born Heinrich Keller; July 11, 1849 – March 10, 1922) was an American magician who presented large stage shows during the late 19th and early 20th centuries.

Kellar was a predecessor of Harry Houdini and a successor of Robert Heller and Isaiah Hughes, under whom he apprenticed. He was often referred to as the "Dean of American Magicians" and performed extensively on five continents. One of his most memorable stage illusions was the levitation of a girl advertised as the "Levitation of Princess Karnac", copied from an illusion invented by John Nevil Maskelyne, which was later bought by Harry Blackstone Sr.

He was a longtime customer of the Martinka Magic Company, which built many of his illusions and sets, including the "Blue Room".

As is the case with most magicians, there is little of Kellar's early life that can be confirmed. His real name was Heinrich Keller and he was born to German immigrants in Erie, Pennsylvania. He was sometimes called Henry, but later changed it to Harry. He also changed the spelling of his last name from Keller to Kellar, to avoid confusion with Robert Heller. As a child, Kellar loved to play dangerous games and was known to play chicken with passing trains.

Kellar apprenticed under a druggist and frequently experimented with various chemical mixtures. On one occasion, Kellar reportedly blew a hole in the floor of his employer's drugstore. Rather than confront the wrath of his parents, Kellar stowed away on a train and became a vagabond. He was only ten years old at the time.

Kellar was befriended by a British-born minister of religion from upstate New York. He offered to adopt Kellar and pay for his education if he would study to also become a minister. One evening Kellar saw the performance of a traveling magician, "The Fakir of Ava", the stage name of Isaiah Harris Hughes, and, after the show, Kellar "immediately got the urge to go on the stage". He later told Houdini that, "I became very restless, bought books on magic and finally left my friend and benefactor".

While working on a farm in Buffalo, New York, Kellar answered an ad in the newspaper that was placed by Hughes, who was looking for an assistant. Kellar was hired at the age of 12, and at the age of 16, he gave his first solo performance in Dunkirk, Michigan; it was a disaster and Kellar went back to work with Hughes. Two years later, Kellar tried again with better results, but, as he was in poor financial condition, his early career often consisted of borrowing equipment for the show and avoiding creditors.

In 1869, Kellar began working with "The Davenport Brothers and Fay", which was a group of stage spiritualists made up of Ira Erastus Davenport, William Henry Davenport and William Fay. With the Davenports and Fay, Kellar learned a lot about how to conduct a successful show. Kellar spent several years working with them, until 1873, when he and Fay parted ways with the Davenports and embarked on a "world tour" through Central and South America.