Coastal salt marshes - once seen as "wasteland" to be reclaimed for farming and industry - play a vital role in offsetting emissions, a study has found.
It estimated Wales' marine environment annually locks away as much carbon dioxide as is produced by 64,800 cars.
Natural Resources Wales (NRW), which commissioned the study, said it was working to restore the sites.
But a sea charity claimed the findings showed more needed to be done.
The Marine Conservation Society said the protection of offshore habitats - out at sea as opposed to along the coast - was lacking.
There are more than three million hectares of so-called blue carbon habitat - such as coastal salt marshes and and seagrass beds - in Wales.
Researchers estimated it already stored at least 113 million tonnes of carbon - equivalent to 10 years' worth of Wales' emissions.
Of the habitats they studied, salt marshes - coastal wetlands that are constantly flooded and drained by the tides - were found to be most effective at storing carbon.
They are home to a variety of plants which capture carbon dioxide from the air and water before exuding it into the sediment below through their roots and are now largely protected as part of special areas of conservation.
Expanding them, or restoring sites once taken over by farms and industry, could increase the amount of carbon stored, the researchers said.
Lily Pauls, senior marine advisor for NRW, said sites such as the north Gower salt marsh, were also vital habitats for wildlife.
"We have over 20 species of salt marsh plant you can find here and there's associated insect life as well.
"It also acts as a nursery for juvenile fish and in terms of birds, we have waders, oystercatchers, knots, dunlins, and shelducks that roost and feed on the saltmarsh."
Other carbon stores include living organisms such as seagrass and the shells of marine animals.
'A lot more to do'
Dr Cai Ladd, an expert in the field at Glasgow University, said NRW's study was important and restoring these areas should be seen as a "key priority".
"There are very interesting processes that happen which are unique to the coast. Sediments accumulate over time so what you end up getting are very deep deposits full of carbon," he added.
Gill Bell, head of conservation for Wales at the Marine Conservation Society said: "We need to invest in blue carbon - at the moment the Welsh Government is focusing its attention on marine renewables and fisheries.
"But what we need to be doing is restoring and repairing all those habitats that can store carbon for a long time, from saltmarshes to the seabed itself.
"For instance we allow trawling in a lot of the seabed and every time we do that we're stirring up all of that stored carbon."
"So we need to have some better protection for all our inshore and offshore marine habitats."
Rhian Jardine, NRW's head of marine services described the findings as "very exciting".
"We have a very extensive range of marine protected areas in Wales and it's really important that we ensure they're effectively managed so that the potential these habitats have for storing carbon is met."
A major project to expand seagrass beds off the Pembrokeshire coast - led by Swansea University, the World Wildlife Fund and Sky Ocean Rescue - is under way.
The Welsh Government said it welcomed the work being done, adding: "We are committed to restoring, improving and enhancing our marine ecosystems, and emerging evidence - such as that provided by NRW - will play a vital role in this work."