Is the Marijuana plant a type of forbidden form of Kitniyos?
Generally, things that do not have the tradition of being kitnyot, are not kitnyot. A good example of this is quinoa. It is similar in many ways to other types of banned kitnyot, yet the generally accepted halakhic authorities do not classify it as kitnyot, partly because it was not included in the original custom. Based on the tradition of kitynot, there is no way marijuana would be defined as kitnyot. Practically speaking, marijuana for recreational purposes would be forbidden (as Barry noted in referencing Reb Moshe's teshuva on it). I haven't heard or read anything about using it for medical purposes.
It seems the answer is yes according to this article which I think has been fetured here before.