The event is being held at Kew’s Millennium Seed Bank at Wakehurst, which has the largest wild seed collection in the world.
Gardeners will be able to swap seeds from plants they have grown at home, and seeds will also be available for a small donation.
Other speakers will include Bob Sherman and Neil Munro from Garden Organic Heritage Seed Library, talking about the importance of gardens for biodiversity, and heritage seed varieties.
Rare varieties of heritage vegetables, such as Duke of Albany peas and red tankard turnips, will be on show in the Wakehurst nursery, which is not normally open to the public.
These varieties are being conserved by national charity Garden Organic’s Heritage Seed Library.
There will also be talks and practical demonstrations on cleaning, drying and storing seeds from experts at Wakehurst.
Head of Kew’s Millennium Seed Bank Dr Paul Smith said: "The Great Seed Swap is a new event for Wakehurst and we hope it will appeal to people already growing their own edible produce – and others interested in getting started.
"Our aim is to increase awareness of the incredible diversity of plant varieties that you can grow at home for food, and to encourage sharing of these varieties through the exchange of seeds. There will be a wealth of expert advice and information on hand to help you make the most of your own plot, whether you have a large garden, a small backyard or even just a windowbox."
The event is supported by Garden Organic Heritage Seed Library and Seedy Sunday (Brighton and Hove). More information is available at www.kew.org/seedswap.