Wikipedia: "The Zohar first appeared in Spain in the 13th century, and was published by a Jewish writer named Moses de Leon. De Leon ascribed the work to Shimon bar Yochai, a rabbi of the second century CE"
Orthodox Jews generally accept that this was indeed the work of R' Shimon bar Yochai, but it seems to be a subject of controversy.
(Shimshon, I think you mentioned this in one derech hashem class.)