We report two cases of ganglion cysts in the posterior septum of the knee joint, one as parameniscal cyst from the posterior horn of the lateral meniscus extending to posterior septum, and the other as a cyst located in the posterior septum adjacent to the posterior cruciate ligament, which were both arthroscopically excised expediently by posterior trans-septal portal. The posterior compartment of the knee is not readily accessible by ordinary arthroscopic portals, and therefore has been considered as 'blind spot' conventionally. The posterior trans-septal portal is useful for assorted diagnostic or manipulative procedures in the posterior compartment of the knee.