Research released by the environmental group found the 13 rivers tested across the UK from the Exe in south-west England to the Lagan in Northern Ireland, found they all had tiny pieces of plastic measuring less than 5mm.
Greenpeace is calling for the Government to introduce "bold" new plastic reduction targets and create an independent watchdog with powers to enforce them.
Scientists and campaigners, including Harry Potter star Bonnie Wright, sampled points along the rivers Exe, Thames, Severn, Great Ouse, Trent, Mersey, Aire, Derwent, Wear, Conwy, Wye, Clyde and Lagan.
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Analysis of the samples by Greenpeace scientists at the University of Exeter using an infrared detector found microplastics were in 28 out of 30 locations tested.
A total of 1,271 pieces of plastic ranging from fragments of straws and bottle tops to tiny microbeads less than 1mm across in size were caught in a specially designed net by the survey team.
The highest concentrations were in the River Mersey, where 875 pieces were captured in half an hour.
This makes the waterway, at the time it was sampled, proportionally more polluted than the great Pacific garbage patch, considered by scientists to be one of the most plastic-polluted expanses of water on Earth, Greenpeace said.
The research also found that microbeads, tiny spherical pieces of plastic often used in cosmetic and household products, were found in five rivers despite being partially banned in 2017.
Seven locations turned up plastic pellets known as "nurdles", which are used in the production of plastic products.
The most numerous instances of microbeads and nurdles were found in the River Mersey.
More than four fifths of the polymers found by Greenpeace were polyethylene, polystyrene and polypropylene, which are used to make products such as food packaging, milk and water bottles and carrier bags.
Fiona Nicholls, ocean plastics campaigner for Greenpeace UK, said: "This study is a wake-up call for Government.
"Fiddling around the edges of the plastic pollution problem by banning straws simply doesn't cut it.
"We need to see bold new plastic reduction targets in the upcoming Environment Bill, and aim to at least halve single use plastic production by 2025."
Steve Backshall, wildlife expert and TV presenter, said: "Greenpeace's study has discovered that the River Mersey is even more polluted than the Great Pacific Garbage Patch - surely this will galvanise us all into doing something about this.
"Plastic pollution isn't just a domestic issue, its impacts are seen on wildlife and humans all over the world.
"For the sake of nature and for the sake of future generations we need to stop producing so much of it - it's the only way forward."
A spokesman for the Environment Department (Defra) said: "The UK is a global leader in tackling plastic pollution and is already making great strides - banning microbeads in rinse-off personal care products, taking fifteen billion plastic bags out of circulation with our 5p carrier bag charge, and announcing plans to introduce a deposit return scheme for single use drinks containers.
"We know there is more to do, which is why we are funding ground-breaking research into how microplastics enter waterways and working with the water industry to find new methods to detect, measure and remove microplastics from wastewater."