Box Jellyfish are poisonous jellyfish that only appear in The Getaway. Greg seen the Box Jellyfish in the show called "World's Most Venomous Creatures", and they had listed it in the top. He said that if you are stung by Box Jellyfish, your heart can stop and then you're pretty much finished. Manny got the Box Jellyfish during snorkelling. When the Heffley Family came back to the Isla de Corales, they rest and then the Director of Fun came through in a conga line trying to join them. He kept circling around them until he stopped cold when he spotted something inside Manny's bucket. Greg said that it looked like a clear plastic bag floating around in Manny's bucket but the Director of Fun lifted the bucket to take a closer look. It turns out it was not a plastic bag. It was actually a Box Jellyfish inside, which he screams and tosses into the pool behind. He realized that the Box Jellyfish inside the pool, so he ran to the nearest lifeguard, who started blowing her whistle. The other lifeguards also blowing their whistles. People frantically trying to get out of pool so fast. The Heffley Family decided to leave the commotion. Later, the pool was drained because the pool workers could not locate the Box Jellyfish, making everyone infuriated at the Heffleys.
- It is unknown how Manny got the Box Jellyfish without getting stung during snorkelling.
- The Box Jellyfish may not have been an actual Box Jellyfish, due to several hints suggesting that Isla de Corales is located in the Carribean, and the fact that Box Jellyfish are native to Australia. If so, either Greg misidentified it, or it was a factual error.
- Diary of a Wimpy Kid: The Getaway (First and only appearance)