As global population increases, the connections between food, water, energy and the environment at global and regional scales become ever more important. The complexity and inter-connectedness of these relationships challenge policymakers, scientists, businesses and citizens to find acceptable ways forward, but there are no easy solutions. This is the ‘nexus’. Citizen science can provide a powerful mechanism to help tackle these environmental and social challenges. In this thinkpiece we draw on the experiences of citizen science practitioners, particularly from the environmental sector. Citizens are the guardians of their local environment and, arguably, often know the places where they live better than regulators, policymakers and industry. Local citizens will usually be the first to notice changes in their immediate environment, whether instant changes (such as a pollution spill) or gradual (such as species decline). Citizen science can generate and broaden out the kinds of data that are considered in the investigation of environmental issues. Benefits of participating in citizen science include raised awareness, increased education, greater involvement, more participatory democracy, and increased ownership of solutions. Participation may also bring wider social, health and wellbeing benefits. Professional scientists in turn benefit from the data submitted by volunteers, the value of which can be estimated at many millions of pounds per year. Some of the generic challenges to successful citizen science will be heightened in the context of understanding and dealing with nexus issues. These include extending citizen science (which is normally conducted at local level) to regional and global scales, optimising the collection of data through better coordination between practitioners, empowering citizens and businesses to take more control of the conception and design of citizen science activities, and understanding the motivations, attitudes and practices of all participants. Citizens are the sleeping giant on whose shoulders scientists must stand. This giant must be awoken and empowered to work with policymakers and businesses – and to hold them to account.