John Patrick O'Brien (February 1, 1873 – September 22, 1951) was an Irish-American politician who served as the 98th Mayor of New York City from January 1 to December 31, 1933.

O'Brien's inauguration was held in the Hall of Records, at 31 Chambers Street in Manhattan, and was devoid of the pageantry that had greeted many of his predecessors.

His inauguration speech did not outline a vision for the city, but rather, reflected on the work of the court and the legal profession in general.

Although he is credited with expanding the city's ability to collect taxes, restoring order to the city's finances, and trimming the budget, O'Brien was defeated for re-election in a three-way race by the colorful Republican-City Fusion Party candidate, Fiorello H. La Guardia, in November 1933.

O'Brien returned to his legal work and served three times as a delegate to the Democratic National Convention.