A soybean cultivar designated CL431203 with high yield potential and tolerance to Roundup.TM. herbicide, further including the plants and seeds of the cultivar CL431203, methods for producing a soybean plant by crossing the cultivar CL431203 with itself or another soybean plant. The invention also relates to soybean cultivar CL431203 further comprising one or more single gene traits, and to methods of producing a soybean having such traits by introgression, transformation or mutagenesis. The invention also includes using the soybean cultivar CL431203 to produce other soybean cultivars, breeding lines, and progeny.