In a statement posted on his official website, Johnson said his views "have evolved sufficiently to support marriage equality legislation."
"This position doesn't require any religious denomination to alter any of its tenets; it simply forbids government from discrimination regarding who can marry whom."
Johnson's statement comes amid polls indicating growing popular support for gay marriage and makes him the 50th sitting senator to support the concept.
Scarlett Johansson steps out with fiance after pregnancy reveal
Jessica Simpson shares three-way kiss with friends in photo