Saying "Jesus was a Socialist" is kinda tough considering Jesus lived centuries before even the roughest concept of Socialism existed.
The question really should be "Are the teachings of Jesus compatible with Socialism?", and I think the answer is a resounding yes.
Just look at what Jesus was preaching.
I. Anti-Consumerism
Jesus is quoted with saying that it's easier for a camel to pass through the eye of a needle than for a rich man to get in to heaven. Over and over, Jesus pointed out that wealth does not equal happiness (the great myth that Capitalism is based on).
II. Solidarity With the Oppressed
Jesus is quoted with saying that which you have done unto the least of my brothers (that is, the poor, the orphans and widows, the imprisoned, etc.) it is as if you have done [it] unto me. In other words, feed the hungry, cloth the naked, defend the weak, etc. This is the direct opposite of what Capitalism teaches.
III. Separation of Church and State
As weird as it sounds, Jesus was the founder of secularism. Take his famous "render unto Caesar (government) what is Caesar's, and render unto God what is God's" speech.
IV. Revolution
The entire reason Jesus was crucified was because of how he angered the established order with his teachings. Jesus was, by any standards, a revolutionary with a clear moral and social teaching.
If you want to go beyond that, the first Socialistic commune in recorded history was founded by the followers of Jesus, who pooled and redistributed their wealth, lived as equals, and took care of each other based on their talents and abilities (see Acts 2:44-45 and Acts 4:34-37).