The Cloud-based enterprise software specialist did not name the customer but said the win is the result of a long-standing relationship with an existing client.
The largest US health insurance companies are WellPoint, CIGNA, Aetna, Humana, United Healthcare and BlueCross BlueShield, though the latter works on a state-by-state basis.
Rosslyn says the deal hands it the status of preferred supplier, with the potential to reach affiliated organisations in more than 30 states.
The firm’s core business involves helping governments and large corporates sift “big data”.
Charles Clark, chief executive, said "One of the reasons we were awarded this nationwide contract is the recent release of RAPid InTouch, which is opening up new and exciting market opportunities for Rosslyn.”
“RAPid InTouch represents a technological jump forward in analytics.
“For the first time, non-technical employees in procurement, finance and marketing have a simple-to-use tool.”
Last month Rosslyn said it had signed up the consultancy arm of one of the 'Big 4' accountancy practices, providing data analysis to the company and its clients.
Shares were down 0.14p to 20p.