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.

[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.

[3] His son John F. O'Brien also was a judge of the New York Court of Appeals.