Lenjan County (Persian: شهرستان لنجان) is in Isfahan province, Iran. Its capital is the city of Zarrinshahr.

After the 2011 National Census several villages merged to become the city of Baghshad.

After the 2016 census, Ashiyan and Khorramrud Rural Districts, and the city of Fuladshahr, were separated from the Central District in the formation of Fuladshahr District.

At the time of the 2006 census, the county's population was 225,559 in 58,232 households. The following census in 2011 counted 246,510 people in 71,028 households. The 2016 census measured the population of the county as 262,912 in 81,101 households.

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

Lenjan County is the location of Isfahan Steel Mill. It is on the banks of the Zayandeh River, the largest river in Isfahan province. Farming is an important source of income, and the most important product in Lenjan is rice.

Hossein Rajaei Rizi is the representative of Lenjan in Islamic Consultative Assembly.

The main route passing through the county is Road 51, connecting population centres to Isfahan and Shahrekord.