Kâsânsây also spelled as Kasansay (Uzbek: Kasansay / Косонсой; Tajik: Косонсой; Russian: Касансай) or simply, Kasan, (ancient Kathan or Great Kushan empire) is a city in Namangan Region, Uzbekistan.

[1] Kasansay is named after the River "Kasan" which flows from high mountains of Kyrgyzstan to Turakurgan District (Namangan Region), the word "soy" in from sugdian-tajik and means a "brook".

There are still remains of ancient Mug Castle of Kushanids in the north part of the city.

In other books is written that "koson" means "big town" or "strong castle".

There are five Secondary Special Education Colleges (SSEC) and one Academic Lyceum in Kosonsoy.