A diffuser baffle for use in an in-line wastewater grease interceptor. The diffuser baffle lies within a grease collecting chamber between the inflow and outflow of a grease interceptor. The diffuser baffle has an open bottom which is sized and shaped respectively to fit into the grease collecting chamber. The diffuser baffle has an end operatively connected to the inflow opening to permit wastewater to flow into the body through the open bottom. The baffle has a downstream end which includes a deflector portion to deflect a wastewater stream in a direction generally into the grease collecting chamber. The body also includes a number of apertures to permit the deflected wastewater stream to pass through the baffle and into the grease collecting chamber, where FOG will separate before waste water exits the chamber through the outlet baffle.