Molluscan shells may display a variety of colors, which formation, inheritance, and evolutionary significance are not well understood. Here we report a new variant of the Pacific abalone Haliotis discus hannai that displays a novel orange shell coloration (O-type) that is clearly distinguishable from the wild green-shelled abalone (G-type). Controlled mating experiments between O- and G-type abalones demonstrated apparent Mendelian segregations (1:1 or 3:1) in shell colors in F₂ families, which support the notion that the O- and G-types are under strict genetic control at a single locus with a recessive o (for orange shell) allele and a dominant G (for green shell) allele. Feeding with different diets caused modifications of shell color within each genotype, ranging from orange to yellow for O-type and green to dark-brown for the G-type, without affecting the distinction between genotypes. A previously described bluish-purple (B-type) shell color was found in one of the putative oo × oG crosses, suggesting that the B-type may be a recessive allele belonging to the same locus. The new O-type variant had no effect on the growth of Pacific abalone on the early seed-stage. This study demonstrates that shell color in Pacific abalone is subject to genetic control as well as dietary modification, and the latter probably offers selective advantages in camouflage and predator avoidance.