Demas Barnes (April 4, 1827 – May 1, 1888) was an American businessman and politician and a United States representative from New York, serving one term from 1867 to 1869.

Barnes moved to New York City in 1849 and entered in the drug business, including Charles Henry Fletcher's Castoria.

Barnes crossed the continent in a wagon and studied the mineral resources of Colorado, Nevada, and California.

Upon returning to New York City Barnes wrote articles and published works concerning his experiences in the United States.

He also started his wholesale drug business in New York City in 1853 and was highly prosperous as a patent medicine manufacturer.