Kamalasai (Thai: กมลาไสย, pronounced [kā(m).mā.lāː.sǎj]; Northeastern Thai: กมลาไสย, pronounced [kā(m).mā.la᷇ː.sǎj]) is a district (amphoe) in the southern part of Kalasin province, northeastern Thailand.

Kamalasai is the location of the ancient city Mueang Fa Daet Song Yang.

There were excavated 130 Bai sema sandstones registered with the Fine Arts Department.

In addition, it is also the location of Phra That Yaku, a local biggest chedi in the Dvaravati period during the 7th–11th centuries.

Kamalasai is divided into eight sub-districts (tambons), which are further subdivided into 111 administrative villages (mubans).