Frederick Denkmann (April 8, 1821 – March 2, 1905) was an American lumber baron based in Rock Island, Illinois.

Frederick Carl August Denkmann was born in Salzwedel in the Kingdom of Prussia, present-day Saxony-Anhalt, Germany and immigrated to the United States.

In one day in 1896 a record 64 rafts passed under the Government Bridge between Rock Island and Davenport, Iowa.

Denkmann and Weyerhäuser formed a new corporation for the two mills and called it the Rock Island Lumber and Manufacturing Company.

The fire destroyed 20 acres (8.1 ha), left 250 people homeless and caused $1.25 million in damage.

[1] By this time Friedrich Weyerhäuser had re-located to the Pacific Northwest where he had recently established the Weyerhaeuser Timber Company.

While Denkmann was still alive the two gave substantial amounts of money to build the Rock Island Public Library.

On April 5, 1976 the cemetery superintendent Joseph Vogele discovered that the mausoleum had been broken into and a stained glass window by Louis Comfort Tiffany was stolen.

It was eventually returned to the Denkmann family in 1997 and a Tiffany glass conservator was hired to repair a couple of cracks in the window in 1999.