A modular container, such as a cooler or ice chest, is provided for storing items. The modular container includes two rigid end modules with, optionally, one or more rigid inner modules. Each module attaches to an adjacent module along mated channels with seals. A locking system engages between each module to resist the separation of one module from another. The mated modules provide for a water tight container sufficient for use as a cooler or ice chest. The size of the container is increased or decreased by adding or removing inner modules, respectively.