The study by Upper Itchen Restoration Community Interest Company and the University of Brighton found plastic and fibreglass particles in the river’s headwaters.
Citizen scientists and researchers detected microfibre and microparticle pollution at all sampled sites in spring 2025.
Dr Corina Ciocan, principal lecturer at the university, said: “Chalk streams like the Itchen are unique and often compared to rainforests for their biodiversity.
“Finding microfibres and even fibreglass fragments at every site we sampled is concerning, but it also gives us the evidence we need to act.
“The next step is to identify the sources of this pollution and work with local partners to reduce it and mitigate the impacts.”
Ian Diver, a director of the UIR CIC, said: “These early results show that microparticle pollution is an issue we cannot ignore.
“There is currently no data on particle pollution in the Upper Itchen.
“This project changes that.”