Officials say it is the same swine 'flu strain as identified in previous cases, of which there are 32 confirmed in the UK.
No further information was immediately available on where in the county the case had been confirmed.
Dr David Walker, director of public health for East Midlands NHS, which covers Northamptonshire, said: "We have been preparing for a possible 'flu pandemic for a number of years and the East Midlands have robust plans in place.
"We would advise patients and the public to be concerned and vigilant but not alarmed.
"The best way to prevent the spread of swine 'flu is for everyone to play their part and follow these simple good hygiene practices.
"Always cover your nose and mouth using a tissue to catch your sneezes, throw away used tissues where germs can linger and wash your hands regularly."
Those who have returned from an affected area, or have 'flu-like symptoms and are worried, are advised to stay at home.
They should look for information on the NHS Choices website, www.nhs.uk, or call the National Swine Flu Information Line on 0800 1513513.
People with further concerns can contact NHS Direct on 0845 4647, where trained health advisors can give specific advice on swine 'flu. Where appropriate, callers will be transferred to nurse advisors who will assess their condition.
Residents can also contact their local GP for advice.