Thumb polydactyly is reported to be the most common congenital anomaly of the hand in Japan. The floating type is not particularly rare, accounting for 0.9 to 15% of all cases of thumb polydactyly. However, to the best of our knowledge, there has been only one case of thumb polydactyly with a floating ulnar thumb, reported by Onizuka. Herein, we report a case very similar to that reported by Onizuka. In our case, the vessels feeding the floating ulnar thumb branched from the superficial palmararterial arch, and X-rays revealed triphalangism. In surgery, we not only reconstructed the morphology of the thumb, but also tried to preserve the sensation in the reconstructed thumb by transposingthe digital nerve of the floating ulnar thumb to the radial thumb. In addition to thumb polydactyly,our case also showed hypoplasia of the thenar muscles.