The Monkey Bank is a Resource Building in Bloons Monkey City. It increases the City Cash capacity of the player's City. Each Monkey Bank increases the capacity by 1,500 at level 1, and starting at City Level 15, the player can upgrade them to increase their capacity. In the Flash version, they have upgrades up to level 8, while in the multiplatform version, they only have upgrades up to level 4. One Monkey Bank is provided at the start of the game, and the player is not allowed to demolish the last Monkey Bank.
Internally, researching Banana Investments Advisory requires building a Monkey Bank first, but because the player always starts with one Monkey Bank and it is impossible to demolish the last Monkey Bank, this research effectively has no building requirement, making it the only tier 4 upgrade with this distinction.