The company, in a statement released on Friday afternoon, admitted "a subset of Epsilon clients' customer data were exposed by an unauthorized entry into Epsilon's email system". It claimed the data "was limited to email addresses and/or customer names only".
Tivo, Best Buy and JPMorgan Chase are among the brands that have alerted customers to the problem.
Epsilon is one of the largest email marketing companies in the world and sends more than 40 billion emails annually on behalf of more than 2,500 clients.
There is no suggestion the problems have affected Epsilon's operations in the UK.
Epsilon's UK office declined to comment and directed all enquiries to its US press office.
A spokesman at Tesco, one of Epsilon's UK clients, said: "Epsilon have said these problems are limited to the United States."