A local charity organising litter picks on Guernsey beaches collected more than 48,000 pieces during 2021.
The Clean Earth Trust (CET) said it analysed 122 beach litter collections, taking volunteers more than 2,000 hours.
The results showed that 48,924 pieces of marine litter weighing more than two tonnes were collected.
Paul Bugden, policy officer of CET, said the report was a "ground-breaking piece of citizen science".
He added that it demonstrated the "shocking impact of marine plastic pollution in Guernsey and globally and is a call for action which we will be addressing with the States of Guernsey".
The report found 76% of the items were plastic and 33% of all items were single-use plastic.
A total of 7,275 cigarette butts were collected.