| Ernest Aloysius O'Brien | |
| Office: | Judge of the United States District Court for the Eastern District of Michigan |
| Term Start: | March 4, 1931 |
| Term End: | October 9, 1948 |
| Appointer: | Herbert Hoover |
| Predecessor: | Seat established by 46 Stat. 1197 |
| Successor: | Thomas Patrick Thornton |
| Birth Name: | Ernest Aloysius O'Brien |
| Birth Date: | 1 July 1880 |
| Birth Place: | Detroit, Michigan |
| Education: | Detroit College (B.A.) Detroit College of Law (LL.B.) |
Ernest Aloysius O'Brien (July 1, 1880 – October 9, 1948) was a United States district judge of the United States District Court for the Eastern District of Michigan.
Born in Detroit, Michigan, O'Brien received a Bachelor of Arts degree from Detroit College (now University of Detroit Mercy) in 1898 and a Bachelor of Laws from Detroit College of Law (now Michigan State University College of Law) in 1905. He was in private practice in Detroit from 1906 to 1928. He was a Judge of the Circuit Court of Michigan in 1928, returning to private practice from 1928 to 1931.
On February 26, 1931, O'Brien was nominated by President Herbert Hoover to a new seat on the United States District Court for the Eastern District of Michigan created by 46 Stat. 1197. He was confirmed by the United States Senate on March 2, 1931, and received his commission on March 4, 1931. O'Brien served in that capacity until his death on October 9, 1948.