Taraxacum officinale (common dandelion; Asteraceae) was first noted as an introduced plant in Japan in 1904 (Makino 1904). Currently, this species grows in plains and mountainous habitats throughout Japan, from Hokkaido to Ryukyu. The present study collected 3,941 individuals from 108 sites (on average, 36.5 plants per population) throughout the Japanese archipelago, and examined their somatic chromosome counts. We found that Japanese T. officinale is a polyploid complex (x=8) composed of triploid (3x) (2n=24) and tetraploid (4x) (2n=32) individuals. We also found that 3x T. officinale is distributed throughout Japan, from Hokkaido to Okinawa, whereas 4x tetraploid T. officinale is limited to Honshu, Shikoku and Kyushu. Karyotypes were examined for 37 individuals that were used in the plants examined for their chromosome numbers (Table 2). This yielded a total of eight forms of 3x T. officinale and four forms of 4x T. officinale. Previously, 2n=24+2B chromosomes and 2n=26 chromosomes have been reported for Japanese T. officinale. However, these were considered miscounts due to erroneous recognition of satellite bodies as two B chromosomes or two chromosomes of the two large satellites in some races of 3x T. officinale with large satellites. Taraxacum officinale has hundreds of apomictic microspecies worldwide. Our study reveals that some races in both 3x and 4x T. officinale can be differentiated according to their karyotypes.