Catamaran, formed from the merger of SXC Health Solutions and PBM Catalyst Health Solutions in 2012, helps the administrators of group healthcare plans reduce their prescription drug costs.
UnitedHealth's offer of $61.50 per share represents a premium of 27 percent to Catamaran's Friday close on the Nasdaq.
Catamaran's stock was trading at $60.50 premarket on Monday, while UnitedHealth's was up nearly 3 percent.
After the deal, UnitedHealth expects to fill more than one billion prescriptions, the companies said.
The deal value is based on Illinois-based Catamaran's total outstanding diluted shares as of Dec. 31.
The transaction is expected to close in the fourth quarter of 2015 and add about 30 cents per share to UnitedHealth's profit in 2016, the companies said.