Shahmirzad (Persian: شهميرزاد) is a city in, and the capital of, Shahmirzad District of Mehdishahr County, Semnan province, Iran. It is in the northern part of the country and on the southern slopes of the Alborz Mountains.

At the 2006 census, its population was 7,273 in 1,860 households, when it was in the former Mehdishahr District of Semnan County). The following census in 2011 counted 8,882 people in 2,625 households, by which time the district had been separated from the county in the establishment of Mehdishahr County. The latest census in 2016 showed a population of 11,191 people in 3,789 households. During the summer, the city's population approaches 40,000 as tourists come for its cool climate and popular gardens.

Shahmirzad has been home to people of diverse ethnic backgrounds, a large group of whom seasonally settled in cities and towns of Mazandaran, such as Babol, Sari, Neka, and Behshahr. During the past decades many Muslim, Bahá'í, and Shahmirzadi Jews, migrated to larger cities in Iran and abroad, most notably San Francisco Bay Area.

The Shahmirzadi language (شامرزایی) is a Caspian language close to Mazandarani.

Shahmirzad's walnut orchard with a size of 700 ha is noted by the UN, Food and Agriculture Organization, as the largest of its kind in the world. Shahmirzadi homeowners are given a proprietary interest in the walnut orchard in proportion to the amount of land they own in the village. Shahmirzad also produces mineral water "Tenab Shahmirzad".

Notable people

• Hossein Fathieh, Mine explorer and University founder in Shahmirzad
• Zabihollah Safa, Iran scholar and professor at the University of Tehran (d. 1999)
• Hají Ákhúnd, Apostle of Baháʼu'lláh, Hand of the Cause in the Bahá’í Faith
• Salman Ghaffari, Iran's Ambassador to Vatican (1986–1990), researcher, religious author & lecturer
• Reza Jafari, Researcher at North Carolina State University and President of Road Safety and Transportation Solutions, Inc. (RSTS, Inc.)
• XPINO A famous rapper in Persian rap