木江界县（越南语：Huyện Mù Cang Chải,Mù Căng Chải），是越南北圻地区安沛省下辖的一个县，位于黄连山脉，大部分地域都在1000米以上，1955年在西北自治区成立后设县，下辖1市镇13社。

Mù Cang Chải is a rural district of Yên Bái Province, in the Northeast region of Vietnam. As of 2003, the district had a population of 42,574. The district covers an area of 1,199 km². The district capital lies at Mù Cang Chải.

Mu Cang Chai is a district of Yen Bai province, it shares the border in the north with Van Ban district of Lao Cai province, neighboring in the south of Muong La district of Son La province, being the west of Than Uyen Lai Chau province, sharing in the east of Van Chan district and Yen Bai province. The district is located at the foot of Hoang Lien Son mountain range, at an altitude of 1,000 m above sea level. Want to get Mu Cang Chai district, traveler will go through Khau Pha Pass that is one of the four imposing passes in the Northwest of Vietnam.