| Denis O'Brien | |
| Office: | Judge of the New York Court of Appeals |
| Term Start: | 1889 |
| Term End: | 1907 |
| Office2: | New York State Attorney General |
| Term Start2: | January 1, 1884 |
| Term End2: | December 31, 1887 |
| Governor2: | Grover Cleveland David B. Hill |
| Predecessor2: | Leslie W. Russell |
| Successor2: | Charles F. Tabor |
| Birth Date: | 13 March 1837 |
| Birth Place: | Ogdensburg, New York |
| Death Place: | Watertown, New York |
| Nationality: |  United States |
| Party: | Democrat |
| Occupation: | Lawyer |
| Signature: | Signature of Denis O'Brien (1837–1909).png |
Denis O'Brien (March 13, 1837 – May 18, 1909) was an American lawyer and politician.
Denis O'Brien was born on a farm near Ogdensburg, New York on March 13, 1837.[1] He was admitted to the bar in 1861, and commenced practice in Watertown. He was elected Mayor of Watertown in 1872.[2]
He was New York State Attorney General from 1884 to 1887, elected on the Democrat ticket in 1883 and 1885.[2]
In 1889, he was elected a judge of the New York Court of Appeals, was re-elected in 1903, and remained on the bench until the end of 1907 when he reached the constitutional age limit of 70 years.[2] [3]
He died from appendicitis at his home in Watertown on May 18, 1909.[3]
His son John F. O'Brien also was a judge of the New York Court of Appeals.