Great Caesar's Ghost, or simply Caesar (possibly Caius Julius Caesar) is one of the ghosts haunting the Haunted Mansion.
On one end of the Grand Hall's Birthday Party table, the ghost of an old man wearing a laurel crown and a toga sits in an elaborate chair, smiling contentedly. This character is identified as the ghost of Caesar and is a play on the then-current expression "Great Caesar's Ghost!", hence his name.
"Great Caesar" can refer to any of the numerous Roman emperors who succeeded to the first famous Caesar, Julius Caesar; it could notably be Emperor Augustus Caesar. However, Julius Caesar was nicknamed "the Bald" in his time due to thinning hair, and the Mansion's Great Caesar's Ghost is bald, suggesting he is probably meant to be Julius.
- Great Caesar's Ghost was first mentioned by Ken Anderson among various historical and famous ghosts he planned to have in the Mansion. He is one of the very few of these characters that Marc Davis retained when he took over the project.
- In the Marc Davis concept art, the character is somewhat plump; meanwhile, the actual animatronic used in the end is very skinny, bordering on skeletal.
- The character has sometimes been thought to be Martin, a character on a tombstone described as an "Old Spartan", by people who weren't aware of the "Cæsar" connection. Regardless of it, Martin is, anyway, not supposed to be an ancient Spartan, but more likely an inhabitant of any of the dozens of towns named Sparta in the United States of America, or even simply someone called "Spartan" because of his stern habits.