System and method embodiments are disclosed to provide mechanisms that allow adaptive transmission time interval (TTI) coexistence in Long Term Evolution (LTE) and fifth generation (5G) cellular systems. In accordance with an embodiment, a method for an adaptive TTI coexistence mechanism includes allocating, by a network controller, a LTE TTI at a first bandwidth. The first bandwidth is smaller than an available system bandwidth and is centered around a carrier frequency at a center of the available system bandwidth. The method further includes broadcasting the first bandwidth in LTE system information messages, allocating adaptive TTIs in the available system bandwidth outside the first bandwidth, and broadcasting adaptive TTI bandwidth partitioning information to adaptive TTI-capable terminals.