EXCLUSIVE

Jake "The Snake" Roberts has a cancerous tumor behind his knee ... and the wrestler says it's slowly growing into his muscle -- so he needs emergency surgery to remove the growth.

Roberts tells TMZ ... he noticed a tumor behind his knee the size of a half dollar last month so he had it tested ... and on Friday doctors told him the test came back positive for cancer.

Jake says he's scheduled to go under the knife Tuesday morning ... and after a few weeks off -- he'll make his return to Jersey Championship Wrestling on March 14th -- cancer, or not.

Jake adds, "If the devil can't defeat me, cancer doesn't stand a chance in hell! Pray for sick children who face this horrible disease 'cause The Snake will be just fine."