Grandpa Jefferson is Rowley's Grandfather and Robert Jefferson's father. He made a cameo appearance in Rodrick Rules in which Rowley's family celebrated his birthday and Greg was invited to. He is unhappy with the flavour of the cake and says that he wants chocolate cake for his next year's birthday, but Greg says that it would only be possible if he is alive the next year. This angers Rowley's father.
- He might be deceased as he was never seen again in the books.