Defining characteristics include a slender, reddish-brown body with a long, narrow head.

Neither gender has a vocal sac, but males develop yellowish-brown nuptial pads and sing during mating season (which lasts from January to March).

R. japonica occurs in Honshu, Shikoku, and Kyushu in Japan to the southern region of China.

Within Japan, it inhabits mostly hillsides and plains and is rarely seen in mountain ranges.

More specifically, the brown frog resides in temperate grasslands, hillsides, plains, rivers, swamps, irrigated land, and seasonally flooded agricultural land.