Kosonsoy also spelled as Kasansay (Uzbek: Kosonsoy / Кoсoнсoй; Tajik: Кoсoнсoй; Russian: Касансай) is a town in Uzbekistan. Kosonsoy in Namangan Province is different from the town of Koson of the Qashqadaryo Province of Uzbekistan. Kosonsoy is named after the River "Koson" which flows from high mountains of Kyrgyzstan to Turakurgan District of Namangan Province of Uzbekistan, the word "soy" in Tajik means "small river".

Kosonsoy is an ancient place, its first settlements date back to times of Kushan Empire. The word "koson" came out from the word Kushan. Kosonsoy was an essential part of Kushan Empire together with ancient city Akhsikent, near Namangan city. There are still remains of ancient Mug Castle of Kushanids in the north part of the city But in other books were written that the "Koson" means "Big town","strong castle".

Kosonsoy has a population about 43,684 inhabitants. The centre of district is town of Kosonsoy. The majority of population in Kosonsoy are ethnic Tajiks.

There are five Secondary Special Education Colleges (SSEC) and one Academic Lyceum in Kosonsoy. SSECs include medical College, pedagogical College, transport and communication College, Technical college and few others. There are about 46 secondary schools in Kosonsoy,two or three of them are Tajik schools and one Russian and Uzbek school.