Some £120m in funding will be made available to rural businesses – generating thousands of jobs in rural communities, she said.
Mrs Leadsom made the announcement at the Oxford Farming Conference on Wednesday (4 January).
Rural businesses would soon be able to apply for the next round of the Rural Development Programme for England (RDPE) Growth Programme, she said.
The programme would help new businesses get off the ground and support existing companies to grow, develop new products and access new markets.
Funding has already benefited dozens of businesses across England, Mrs Leadsom told conference delegates.
Speaking afterwards, she said: "A quarter of England's businesses are based in the countryside and this funding will give rural start-ups, family-run businesses and farmers looking to diversify the boost they need."
The RDPE had already supported a range of projects, from installing cutting-edge equipment to restoring flood plains.
Mrs Leadsom said: "The next round will help create more jobs, sell more products and help us access new markets."
As well as boosting businesses and creating jobs, past RDPE projects have benefited the natural environment.
Beneficiaries include Dovecote Farm in Northamtonshire, which has helped restore flood-plain meadows and grassland along the Nene Valley.
Confirmation of the £120m follows a government guarantee that project agrements signed before the UK leaves the EU will be funded for their lifetimes.
However, schemes must be deemed good value for money and in line with the government's "domestic strategic priorities".
The £120m fund will sit alongside recently announced funding for other RDPE projects, including woodland creation and a flood action facilitation fund.
Defra said overall the RDPE would help protect 2.5m hectares of farmed land, create 6,750 rural jobs and contribute to planting of 11m trees.