The National Laboratory Service provides environmental testing and analysis for air, water and soil pollution.
Altrincham-based Rumpus will handle marketing and PR for a four-year period.
The four NLS laboratories based in Leeds, Llanelli, Nottingham and Starcross, will be part of Rumpus' remit.
Rumpus will work with the NLS on forthcoming projects for 2011, including trade exhibitions such as Sustainabilitylive and raising the online profile of the NLS.
Rumpus director Gareth Clements said: 'This win is a great opportunity and an exciting account for the team. It is imperative that we deliver an exceptional ROI for the Environment Agency.'