Work has now started on lowering the seeds onto the seabed off Pembrokeshire to create a new 20,000 sq m (215,280 sq ft) meadow.
Scientists hope it will also help boost fish numbers and support marine wildlife.
Seagrass, which is found in shallow waters of coastal regions, has been declining globally at a rate of about 7% a year since 1990.
That is a result of long-term development of our coastlines and pollution of the sea, according to project leader Dr Richard Unsworth, of Swansea University.
“It is not that we can blame one person, industry or organisation, it’s the growth of a population around the coast,” he said.
Last summer, 750,000 seeds were gathered from sites around the British coast and stored at the laboratories in Swansea University.
The seeds have been transferred into small hessian sandbags and lowered onto the seabed.
Another 250,000 seeds will be gathered later this year and added to the meadow in November.
“We see seagrass as this wonder plant because of its ability to fight climate change, to help fish stocks, coastal communities and livelihoods,” said Alec Taylor of WWF.
“We need to expand hundreds of thousands of hectares of seagrasses, saltmarshes and other coastal ecosystems to avoid some of the damages from climate change.”