| Robert Johnston McMillan | |
| Office: | Judge of the United States District Court for the Western District of Texas |
| Term Start: | January 19, 1932 |
| Term End: | October 27, 1941 |
| Appointer: | Herbert Hoover |
| Predecessor: | DuVal West |
| Successor: | Walter Angus Keeling |
| Birth Name: | Robert Johnston McMillan |
| Birth Date: | 21 August 1885 |
| Birth Place: | Galveston, Texas |
| Death Place: | San Antonio, Texas |
| Education: | University of Texas School of Law (LL.B.) |
Robert Johnston McMillan (August 21, 1885 – October 27, 1941) was a United States district judge of the United States District Court for the Western District of Texas.
Born in Galveston, Texas, McMillan received a Bachelor of Laws from the University of Texas School of Law 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 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 Judge 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 on October 27, 1941, in San Antonio.