Denis O'Leary (January 22, 1863 – September 27, 1943) was an American educator, lawyer, and politician who served as Queens County district attorney and for one year as a member of the United States House of Representatives from New York, from 1913 to 1914.

O'Leary was born on January 22, 1863, in Manhasset, Long Island, New York.

O`Leary was admitted to the bar the same year and commenced practice in New York City.

He was assistant corporation counsel of New York City in 1905 and 1906 and commissioner of public works of Queens Borough in 1911 and 1912.

O`Leary was elected as a Democrat to the Sixty-third Congress and served from March 4, 1913, until December 31, 1914, when he resigned.