Madison-based Summit Credit Union says it and other credit unions are incurring large costs because of the breach.
The lawsuit was filed this month in federal court in Atlanta, where Equifax is based. It seeks class-action status on behalf of more than 5,700 credit unions.
Summit is being represented by the Minneapolis-based law firm Robins Kaplan, which is seeking to be declared counsel for the entire class. The law firm says it believes Summit is the first U.S. financial institution to sue over the breach.
The lawsuit seeks unspecified damages and an order for Equifax to improve its data security.