One who advocates or practices the doctrines of socialism.

a political advocate of socialism

of or relating to or promoting or practicing socialism; "socialist theory"; "socialist realism"; "asocialist party"

advocating or following the socialist principles; "socialistic government"

a theory that places the means of production and distribution in the hands of the community

an advocate of socialism, which is a theory of social organisation that advocates vesting of ownership of land and capital in the community as a whole