The fetes will raise funds for the National Domestic Violence Helpline, which is jointly run by Women's Aid and Refuge.
The markets, which will be hosted in several cities, will offer vintage pieces and items donated by designers.
The retailer will also encourage customers to hold fundraising sales of second-hand fashion items.
In a joint statement, Sandra Horley, chief executive of Refuge, and Nicola Harwin, director of Women's Aid, said: "Our partnership with Topshop has enabled us to reach out to thousands of women. The fashion sale is a brilliant way to get customers involved."
Topshop's website will feature a meter throughout the month, indicating how much money has been raised.