Robert Johnston McMillan (August 21, 1885 – October 27, 1941) was a United States federal judge.
Born in Galveston, Texas, McMillan received an LL.B. from the University of Texas in 1906 and entered private practice in San Antonio, Texas. He was an Assistant general attorney of the St. Louis-Brownsville & Mexico Railroad in Kingsville, Texas from 1907 to 1912. He was a city attorney of San Antonio from 1917 to 1920, thereafter returning to private practice in San Antonio until 1931.
On December 15, 1931, McMillan was nominated by President Herbert Hoover to a seat on the United States District Court for the Western District of Texas vacated by DuVal West. McMillan was confirmed by the United States Senate on January 12, 1932, and received his commission on January 19, 1932. McMillan served in that capacity until his death, in 1941, in San Antonio.