A side underride guard assembly for a trailer has a plurality of support subassemblies coupled to first and second longitudinal main beams of the trailer. Each support subassembly has at least one cross member having a first end adjacent a first longitudinal side edge of the trailer and an opposite second end adjacent an opposite second longitudinal side edge of the trailer. Each support subassembly also has at least one vertically oriented plate coupled to the at least one cross member. A first longitudinal guard member is coupled to the first end of each of the cross members of the plurality of support subassemblies. A second longitudinal guard member is coupled to the second end of each of the cross members. The first and second longitudinal guard members are located below the first and second longitudinal side edges of the trailer, respectively, to prevent or inhibit side underride.