A new phytoseiid mite species, Amblyseius marunumus sp. n., is described and illustrated based on female and male specimens collected from Gunma, Japan. The new species belongs to the americanus species-group, due to the presence of setae J2 and Z1, and having setae z4 longer than z2. Amblyseius marunumus sp. n. is morphologically close to A. yadongensis Wu, but differs conspicuously in the length of the dorsal idiosomal setae z4, r3, S2, S5 and the shape of spermatheca.