Kosów Lacki [ˈkɔsuf ˈlat͡skʲi] is a town in Sokołów County, Masovian Voivodeship, Poland, with 2,152 inhabitants (2004).

Within the municipal territory are the two Treblinka camps.

• 1202 - First mention of a village Kossów found in documents belonging to prince Konrad Mazowiecki.
• 1723 - A permit was granted to organize a market twice a week and a fair four times in a year. Kosów was established as a town.
• 18th century - There were two schools in Kossów, in which 40 percent of the students were girls.
• 1869 - Civic rights were canceled, because Kosów participated in The January Uprising.
• 1964 - A painting by El Greco titled "Ecstasy of St. Francis of Assisi" was found in the presbytery in Kosów Lacki.
• 01.01.2000 - Restoration of civic rights.

• Official town and commune webpage
• The diocesan museum in Siedlce (when is a El Greco picture ) webpage
• Jewish Community in Kosów Lacki on Virtual Shtetl