The Big Blooper is a giantic Blooper that even takes up most of the background.
As Mario, Princess Peach, and Bowser swim through the Tile Pool in order to reach Fort Francis to rescue Tippi, the Big Blooper suddenly appears upset that there are trespassers. The Big Blooper then traps off the heroes and engauges them in battle. After the heroes defeat the Big Blooper, it swims away never be seen again.
In battle, the Big Blooper mainly attacks by using its tentacles to strike one of the heroes.
As according to Barry the Pixl, the way to defeat the Big Blooper is to roast the red tentacle. Bowser's fire breath is recommended as it can strike the red tentacle from a distance. The red tentacle appears every time the Big Blooper attempts to hit one of the heroes twice with its regular tentacles (meaning the red tentacle will appear on the third attack). The Big Blooper's red tentacle must be hit a number of times (usually three) in order to defeat it.
- Big Blooper is arguably the largest Blooper in the series even out-sizing the Super Blooper from Paper Mario and Gooper Blooper from the other games.
- The theme that plays when Big Blooper appears (named Big Blooper Appears) is a remix of the underwater theme from Super Mario Bros. 3.