At a UN oceans summit, delegates from China, Thailand, Indonesia and the Philippines said they would work to keep plastics out of the seas.
Some of the promises are not yet formalised and environmentalists say the measures proposed are not nearly urgent enough.
But UN officials praised the statement.
Meeting in New York, they said it was part of a clear international shift against ocean pollution.
Eric Solheim, the UN's environment director, told BBC News: "There are quite encouraging signs, with nations taking the ocean much more seriously. Of course, there is a very long way to go because the problems are huge."
It is estimated that 5-13 million tonnes of plastics flow into the world's oceans annually. Much of it is ingested by birds and fish â€“ and fragments of plastic have even been found in organisms at the bottom of the ocean.
A recent paper said much of the marine plastic often originates far from the sea â€“ especially in countries which have developed consumer economies faster than their ability to manage waste.
The  Helmholtz Centre in Leipzig, Germany, estimated that 75% of land-borne marine pollution comes from just 10 rivers, predominantly in Asia.
Reducing the plastic loads in these rivers by 50% would reduce global plastic inputs by 37%, it said.
Tom Dillon from the Pew Charitable Trusts, which campaign on oceans, urged China to move quickly.
He told BBC News: "For thousands of years the Maritime Silk Road was a pathway for export of Chinese culture and influence. Will the ocean be a vehicle for export of Chinese pollution, or a new culture of conservation and sustainability?"
A report to the UN conference from the Thailand government says most marine plastic debris is land-based, caused by inefficient waste management and poor handling of plastic wastes.
In Thailand, the total amount of garbage finding its way into the sea was estimated at 2.83 million tonnes in 2016 - of which 12% was plastic.
The Thai government says the nation has established a 20-year strategy to tackle the problem, including developing financial incentives for keeping plastic out of the sea and encouraging eco-packaging design and eco-friendly substitutes for plastics.
In Indonesia, the government is starting a mass education programme for schoolchildren, and in the Philippines new laws are being developed.
Part of the challenge is finding substitutes for plastics. An international prize for smarter materials and design for packaging was launched recently by the Ellen MacArthur Foundation.
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Nations responsible for much of the world's ocean plastic pollution have promised to start cleaning up their act.