Abstract: A modular cooler that is made of individually insulated modular cooler compartment units that can be stacked together to make a custom sized cooler for any occasion needed. The basic unit is a cylindrical bottom container having specific interlocking structure on its top edge. A tubular compartment unit having mating interlocking structure on its bottom edge is detachably engaged with the interlocking structure of the bottom container unit. A locking ring surrounds both of these interlocking structures and by rotating it a short predetermined distance the two units can be locked together or unlocked. One form of locking ring has a solid central wall that forms it into a compartment divider. Additional units can be stacked together by using additional tubular compartment units and locking rings. A disc-shaped top cover has interlocking structure on its bottom edge. O-rings are positioned between the interlocking structures to make the adjacent compartment watertight and airtight.