Mrs Leadsom was speaking at a rural conference held by the Country Land and Business Association in London on Tuesday (6 September).
Brexit was a "huge opportunity," saidMrs Leadsom, who campaigned for the UK to leave the EU in the run-up to the referendum on 23 June.
But she added: "It's vital we provide as much certainty as we can for the rural sector."
Business confidence and certainty was crucial to fulfilling the opportunity – and rural entrepreneurs needed a stable landscape to move forward with growth and investment plans.
The government had already reassured the farming sector that it would receive the same level of agricultural support until 2020.
Chancellor Philip Hammond had also confirmed the government's commitment to funding the Rural Development Programme.
Many rural businesses see planning constraints as one of the biggest barriers to investment.
Mrs Leadsom said the government was preparing to make an announcement on the issue.
"We need a planning system that matches the ambition of rural businesses, and I can assure you this is something I take really seriously," she said.
"The rural productivity plan committed the government to review planning constraints.
"We will be announcing some very positive next steps in the housing white paper which will include details relevant to rural businesses.
On broadband, Mrs Leadsom said the government was committed to providing superfast broadband to 95% of the country's premises by the end of 2017.
She claimed that "excellent progress" had been made since 2010 but acknowledged that extra support was needed to help those in the last 5% or so-called 'not-spot' areas.
The new Universal Service Obligation – giving people a legal right to request a connection speed of 10 megabits per second – went further than any country in Europe, she added.
In addition, the government was introducing 100% rural rate relief, which meant small businesses working hard in their communities would no longer have to pay business rates.
"We're also going further than ever before to improve our resilience to flooding, which as we saw this time last year is a significant threat to your livelihoods."
Even a small increase in productivity has the potential to add billions to rural Gross Value Added nationally, said Mrs Leadsom.
"That's why the government wants to provide the right conditions to enable you to grow your businesses even further, and plan and prepare for the future with confidence."
CLA president and rural business owner Ross Murray said it was important that the government delivered a better deal for the countryside.
"For far too long, government policy has allowed a focus on business support and infrastructure spending in our towns and cities to undermine the focus needed on promoting growth and investment in our countryside," he said.
"Never has it been more important to address this imbalance than today as the rural economy prepares for the potentially seismic changes of Brexit."
More than 400 rural business owners attended the conference – held as new CLA research suggested that rural businesses are investing more than £13 billion a year.
Mr Murray said: "Businesses based on rural land are vibrant, full of entrepreneurial energy and looking to the future.
The research showed there were great opportunities for these small family businesses to boost the rural economy, said Mr Murray.
"They are up for the challenge, but they face an uphill struggle if Government doesn't start doing more to champion this type of rural investment."
The full speech by Defra secretary Andrea Leadsom can be viewed here.