Mangasar Magurditch Mangasarian (December 29, 1859 – June 26, 1943) was an American rationalist and secularist of Armenian descent.

Born in Mashger (now within Turkey) in the Ottoman Empire, he attended Robert College in Constantinople, and was ordained as minister in Marsovan in 1878.

In 1892 he became leader of the Ethical Culture Society of Chicago, a group established by Felix Adler.

In 1900 he organized the Independent Religious Society of Chicago, a rationalist group, of which he remained pastor until 1925.

His books and essays were translated into French, German, Spanish, and other foreign languages.