The new gardens will include an allotment to demonstrate low-cost gardening, a biodiversity garden, with a focus on encouraging wildlife, and a sustainable garden with an emphasis on organic, pesticide and herbicide-free gardening.
They will be created by local volunteers and staff working for Letchworth Garden City Heritage Foundation, a charity which manages the Letchworth Garden City estate and the RHS on existing open green space.
Foundation garden designer Elizabeth Towler will draw up the plans which will feature a number of information boards to help amateur gardeners.
If successful it is hoped they will become a model for community food-growing, especially in low-income areas.
RHS head of regional development, Andrea Van-Sittart said: "This is such an exciting project and a great opportunity to work with the Heritage Foundation to build support for community gardening groups and volunteers, through to creating the gardens and providing training in the form of educational seminars.
"One of our key aims is to reach out and support grassroots gardening here in the UK and although this is still at a very early stage, we hope that we can apply some of our knowledge and resources to make the Letchworth programme a great success."
Part of the original garden city concept was creating a community that could feed itself. The first stage of the project is planned for next year.