A modular plastic building block for constructing a continuous wall of a structure is provided. The modular block has body that is rectangular or L-shaped depending on the position of the block within the desired structure. Each end of the modular block has a connecting member, either a male or female connecting member configured to connect to a respective female or male connecting member of an adjacent modular block. The modular block includes a pair of locking channels having at least one locking groove enabling a user to lock adjacent modular blocks vertically using a locking member. The modular plastic building block is designed for inexpensive building construction, while still providing efficient insulation and a sturdy base due to the density of plastic as the structure is mostly solid. A conduit may be provided to allow for electrical and plumbing connections within the blocks. In some instances, recycled plastic is used.