The rotating disc aerator is a portable aeration device utilizing atmospheric air, rather than using a compressor for its air supply. The aerator includes a motorized base having a rotary shaft and diametrically opposed, oppositely reciprocating rods extending therefrom. A plurality of discs are disposed along the shaft and are connected to the ends of the rods. The discs are free to slide along the shaft as the rods reciprocate, but rotate with the shaft. A plurality of resilient absorbent elements (sponges, etc.) are installed along the rotary shaft between the discs. The aerator assembly is buoyant, about one-half of each absorbent element extending above the surface of the water when the aerator is placed therein. The upper portions of the absorbent elements absorb air as they rotate above the surface, and are compressed to expel the air into the water as they rotate beneath the water surface.