Green Alliance won the 2009 Public Affairs News (PAN) Think Tank of the Year Award in a ceremony last night.
The Alliance, which specialises in environmental issues, beat strong competition in the field from a shortlist including Demos and the Taxpayers' Alliance.
Stephen Hale, director of Green Alliance, said: "It is a gratifying recognition of our work and our ambitions for the future.
"This award will boost our ability to achieve our goals on political leadership, climate change, the sustainable economy and the other themes of our work," he added.
Ian Hall, editor of UK Public Affairs News, said of Green Alliance's win: "Its work is certainly affecting the politics on climate change. The think-tank's events are now firmly established as key moments in the politics of the environment."
The PAN Awards are now in their fifth year.
Previous winners include the Overseas Development Institute and the Local Government Information Unit.