Joseph Samuel Christian Frederick Frey (born Joseph Levi; September 21, 1771–1850) was a German activist who sought to convert Jews to Christianity. He was active in London and in the United States.

He was born in Maynstockheim, Franconia, the third of ten children. He was raised in an orthodox Jewish home. He converted to Lutheranism on May 8, 1798. He subsequently became an activist, seeking to convert other Jews to Christianity. He was active in London before migrating to New York. His writings portrayed Jews very negatively.

In 1809 he founded the London Society for promoting Christianity amongst the Jews after disagreements with the London Missionary Society.