Qazvin County (Persian: شهرستان قزوین) is in Qazvin province, Iran. Its capital is the city of Qazvin.

Persians, Azeris and Tats are the largest ethnic groups in Qazvin County. According to some sources, the majority of people in northern Qazvin (Alamut) are Tats who speak a dialect of the Tati language. However, other sources claim that the majority of people in Alamut are Mazandarani or Gilaks who speak a dialect of the Mazandarani language or Gilaki language.

According to some linguists, the term "Tati" was used by Turkic speakers to refer to non-Turkic speakers. This could explain why some sources claim the people of Alamut are Tats, while others claim they are Mazanderani or Gilaks. Likely, the "Tats" of Alamut are Mazandarani or Gilak speakers who have been labeled as Tats, as historically they were considered Mazandarani or Gilaks.

At the time of the 2006 National Census, the county's population was 530,961 in 142,781 households. The following census in 2011 counted 566,773 people in 169,078 households. The 2016 census measured the population of the county as 596,932 in 188,460 households.

Qazvin County's population history and administrative structure over three consecutive censuses are shown in the following table.

Media related to Qazvin County at Wikimedia Commons