日本茨藻（学名：Najas gracillima）为茨藻科茨藻属下的一个种。

Najas gracillima, the slender waternymph, is a submerged aquatic plant species found in lakes and streams. It is native to China (Fujian, Guangxi, Guizhou, Hainan, Hebei, Hubei, Jiangxi, Jilin, Liaoning, Nei Mongol, Taiwan, Yunnan, Zhejiang), Russian Far East (Amur and Khabarovsk), Japan, Korea, Taiwan, Iran, Alberta, Ontario, Newfoundland, Nova Scotia, New Brunswick, the eastern United States (every state east of the Mississippi River except Florida, plus Minnesota, Iowa, Missouri and the District of Columbia). It is also considered introduced and naturalized in France, Spain, Italy and California (Plumas and Tehama Counties).