A drain assembly for a sink has an abutment surface (14) for abutting an underside of the sink around the drain aperture, and a first wall portion (16) circumscribing an upper drain volume (18). A down-flow conduit (20) and an up-flow conduit (24) are delineated by a second and third wall portions (22, 26), one of which has an edge defining a lip (28) between the down-flow conduit (20) and up-flow conduit (24). An outlet (30) is connected to up-flow conduit (24). A removable lower cover (32) attaches to the wall portions so as to complete a flow path from the down-flow conduit (20) to the up-flow conduit (24) so as to form a trap. At least the first, second and third wall portions (16, 22, 26) and the outlet (30) are integrally formed as a unitary body, most preferably by an injection molding process.