Dell bought Salesforce.com software for 15,000 of the computer maker's employees, the business software company said.
Salesforce Chief Executive Marc Benioff announced the deal at a marketing event in San Francisco.
The company sells it programs, which help businesses manage sales and marketing, as a service that runs on Salesforce computers and can be accessed via only the Internet. Most other software companies sell programs that customers load onto their own servers.
Most Salesforce customers are medium-sized companies. Analysts have said that the company needs to boost sales to large enterprises like Dell to accelerate its revenue growth.
The size of the sale to Dell is the same as one to Cisco Systems, which until now has been Salesforce's single largest customer.
Other big customers include Automatic Data Processing, with 6,700 subscribers; Nokia, which has 5,500, and Merrill Lynch, which has 5,200.