Robert Denniston (October 15, 1800 in Blooming Grove, Orange County, New York – December 2, 1867 in Salisbury Mills,Orange County, New York) was an American lawyer and politician.
He was the son of James Denniston (ca. 1770–1825) and Prudence Morrison Denniston. On September 24, 1823, he married Julianna Howell (d. 1825). Afterwards he married Mary Scott, and they had five sons and six daughters.
He served as an officer of the New York State Militia and as a Justice of the Peace in Blooming Grove. He was appointed by Governor William L. Marcy a judge of the Court of Common Pleas of Orange County
He was a Democratic member of the New York State Assembly (Orange Co.) in 1835, 1839 and 1840.
He was a member of the New York State Senate (2nd D.) from 1841 to 1847, sitting in the 64th, 65th, 66th, 67th, 68th, 69th and 70th New York State Legislatures. In the Senate, he was Chairman of the Committee on Canals.