Frederick Crayton Ainsworth (September 11, 1852 – June 5, 1934) was an American surgeon and military officer who was Adjutant General of the United States Army.

A gifted administrator, Ainsworth revolutionized government record-keeping methods following his initial appointment in 1886 to the Record and Pensions Division of the War Department, making them vastly more efficient.

In the later stages of his career, Ainsworth, once an innovator, became resistant to further reform proposed by the civilian leadership and was eventually forced from office in 1912 with the threat of court-martial for insubordination.

[2] He resigned his medical corps commission and was appointed a colonel with continued duty as head of the Records and Pension Office on May 27, 1892, and chief of the same with the rank of brigadier-general on March 2, 1899.

Ainsworth died in Washington, D.C., on June 5, 1934, and he was buried at Arlington National Cemetery, Section 3, Lot 1389.