The fund was announced by Theresa May ahead of the Commonwealth Heads of Government meeting in London next week.
The Prime Minister is due to ask all the 52 leaders at the meeting to sign up to the Commonwealth Clean Oceans Alliance set up to help developing Commonwealth countries research and improve waste management.
So far four Commonwealth countries, New Zealand, Sri Lanka, Vanuatu and Ghana, have joined the UK in the alliance.
The Government said £25m of the fund is available to help researchers investigate the issue of marine plastic from a scientific, economic and social perspective.
Another £20m has been earmarked to curb plastic and other environmental pollution generated by manufacturing in developing countries and prevent it seeping into oceans.
The Government pledged to match public donations to tackle plastic waste through the UK Aid Match up to a total of £5m.
The PM said: “This week we will look closely at how we can tackle the many threats to the health of the world’s oceans, including the scourge of marine plastic pollution.
“As one of the most significant environmental challenges facing the world today it is vital that we tackle this issue, so that future generations can enjoy a natural environment that is healthier than we currently find it.
“The UK public has shown passion and energy in the fight against plastic waste, and I believe the Commonwealth is uniquely placed to further this transformative action.
“It is a unique organisation with the strength and the commitment to make a difference.”
She added: “If we stand together, we have the opportunity to send not only a powerful message to the world, but also to effect real change.”
Britain, which is co-chairing the event with Vanuatu, will ask Commonwealth nations to follow the UK’s lead in banning microbeads and slashing the number of single use plastic bags.
WWF chief executive Tanya Steele said: “This alliance, and the leadership the UK Government is showing through the Commonwealth, demonstrates that we’re committed to being part of a global solution.
“Two billion people around the world lack access to effective waste collection, so much of the plastic they use ends up in our oceans.”
“Devoting UK international development money to help poor communities clean up and better manage their waste isn’t just good for nature, it’s good for people too.”
Sky’s Ocean Rescue campaign encourages people to reduce their single-use plastics. You can find out more about the campaign and how to get involved at www.skyoceanrescue.com