Big will support SNH's in-house PR team in promoting the positive contribution nature can make to the health and wellbeing of individuals and economic growth.
A core focus of Big's activity will be on highlighting nature and green space in urban areas and generating more feature-based articles in the print and broadcast media.
SNH press and PR manager Calum MacFarlane said: 'We have a vital role to play in helping people understand, appreciate and enjoy nature, much of which is often on their doorstep.'
The SNH contract is the latest win for Big's public sector division, which was formed recently to focus on the sector.
Big account director Kim Munro, who leads the division, said: 'We'll work alongside SNH's in-house team to complement and add value to ongoing activities and provide support to fully maximise media exposure.'