Marcellus Monroe Crocker (February 6, 1830 – August 26, 1865) was a general in the Union Army during the American Civil War most noted for his service in the Western Theater.

He became very ill en route to join the army of William T. Sherman preparing for the Atlanta Campaign and tendered his resignation on May 14, 1864, but it was not accepted by Secretary of War Stanton as "the department is unwilling that the Country should lose the service of so valuable an officer as General Crocker.

"[3] Officially relieved at Santa Fe on March 1, 1865, and ordered to report to General George H. Thomas at Nashville, Crocker arrived in St. Louis on April 21, 1865, but was again so ill that he could not proceed to his new assignment with the Army of the Cumberland and he once more asked to be mustered out of service.

Granted a ninety-day leave of absence, he was ordered to personally report on matters in the Southwest directly to the War Department as soon as his health allowed.

[4] In late summer Crocker reported to the War Department, but his health continued to decline, and he died at Willard's Hotel on August 26, 1865, in Washington, D.C.[5] He is buried in Woodland Cemetery in Des Moines, Iowa.