Master Joe (マスター・ジョー, Masuta Jō) was a tiger level monster who was the Treasurer of the Monster Association (怪人協会金庫番).
A human sized monster who wore a hoodie with "Joe" written on it and a mask-like face. On his face he had one eye and a mouth, they joined together to look like a key hole. He had a long black tail with three hoops at the end, resembling a key handle. He carried many keys with him, one resembling a skull that he wore as a chain around his neck, and multiple key chains attached to his waist. He also had black shorts and running shoes.
Master Joe is present during the raid on the Monster Association Headquarters, standing guard near the vault with a group of monsters. They are all killed by Atomic Samurai's disciples and the remnants of Narinki's Private Squad, so he rushes to open a large safe. He gets shot by Tongara, but manages to open Evil Natural Water's tank and release it to wreak havoc. He is killed by Mad Doctor Fish in the ensuing chaos.
- His name is a pun based on the Japanese word for key: 錠 (Jō)
- His name also appears to be a reference to the popular lock brand, Master.