He never endorsed socialism as full-throatedly as Einstein, but yes, he was a socialist. The Poor Peoples’ Campaign was an attempt to build working class solidarity. I don’t think he ever centered the idea of socialism in his writings or speeches but it was a recurring theme. A couple of famous quotes:
I imagine you already know that I am much more socialistic in my economic theory than capitalistic… [Capitalism] started out with a noble and high motive… but like most human systems it fell victim to the very thing it was revolting against. So today capitalism has out-lived its usefulness.
We must recognize that we can’t solve our problem now until there is a radical redistribution of economic and political power… this means a revolution of values and other things. We must see now that the evils of racism, economic exploitation and militarism are all tied together… you can’t really get rid of one without getting rid of the others… the whole structure of American life must be changed. America is a hypocritical nation and [we] must put [our] own house in order.
There must be better distribution of wealth and maybe America must move toward a democratic socialism.