Wade Robson is going after another Jackson family member -- this time threatening legal action against Joe Jackson if he continues to spread vicious lies ... this according to legal docs.
Wade's lawyers fired off a cease and desist letter to Papa Jackson on May 25, accusing him of slander and warning JJ to stop dragging Wade's name through the mud.
The letter cites a video TMZ posted ... in which Joe tells our photog Wade is being PAID to lie about the whole "Michael Jackson molested me" thing ... as part of some giant conspiracy.
Wade's lawyers unload on Joe for the accusation, writing, "Your statement is particularly reprehensible in light of the fact that [Wade] has only recently been able to come forward which is so typically characteristic and tragic for so many survivors of sexual abuse."
It continues, "Any trier of fact would conclude your statement was malicious, intended to further injure our client."
Then come the threats ... Wade's lawyers say they will not hesitate to "take any and all appropriate legal measures to vehemently protect the truth and commence legal action."
Translation -- giant lawsuit.