Franklin P. Geyer (July 28, 1853 – October 4, 1918) was an American police detective from Philadelphia, Pennsylvania, best known for his investigation of H. H. Holmes, one of America's first serial killers.

Geyer was a longtime city employee of the Philadelphia Police Department, and in 1894 was assigned to investigate the Holmes-Pitezel Case.

He published the story in his book The Holmes-Pitezel Case: a history of the greatest crime of the century and of the search for the missing Pitezel children.

"[8] While Holmes' "Murder Castle" is a total fabrication, it is true that he killed multiple times, partly in furtherance of an insurance fraud scheme.

[9][10][11] Philadelphia city detective Frank Geyer was tasked with investigating and the trail led him through the Mid West and Toronto, Canada, where he found the remains of two of the Pitezel children.

He continued his search and found the burnt remains of Howard Pitezel, the third child, in a house Holmes had rented in Irvington, Indianapolis.

Modern thought links Holmes to the murders of Ben Pitezel and his three children, as well as very possibly (though by no means unquestionably) to five women he had various personal and business dealings with in the late 1880s and early 1890s, and who disappeared at various points and were never found.

[17] Several popular books falsely claimed Detective Geyer's wife and twelve-year-old daughter died in a fire shortly after he was assigned to investigate H. H. Holmes and the three missing Pitezel children.

He authored the Holmes-Pitezel case: a history of the greatest crime of the century and of the search for the missing Pitezel children, which became an instant best seller.