Grandpa Jefferson is Rowley's Grandfather and Robert Jefferson and Rowley’s Uncle’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.
- Diary of a Wimpy Kid Online (First appearence)
- Diary of a Wimpy Kid: Rodrick Rules (First and only book appearance)
- He might be deceased as he was never seen again in the books.