Thousands of people took to the streets of the capital, Freetown, at the stroke of midnight – marking 42 days without a single declared case of the disease.
There were further cheers when the WHO local representative made the official announcement later on Saturday.
The outbreak killed almost 4,000 people in Sierra Leone over the past 18 months.
Many gathered around a giant cotton tree in the centre of the city. Some lit candles in memory of the victims, while others danced with joy.
Dr Oliver Johnson, from the King’s Sierra Leone partnership, worked at an Ebola clinic in Freetown, and has strong links with medical professionals there.
“[For] everyone I’ve spoken to, there’s a sense of relief that this might finally be over and maybe a bit of disbelief that after so many false starts, or false ends, we might finally be there,” he told the BBC.
A country is considered free of human-to-human transmission once two 21-day incubation periods have passed since the last known case tested negative for a second time.
Sierra Leone President Ernest Bai Koroma is due to address crowds in the city later,
On Friday, he blamed the WHO for delaying Sierra Leone declaring a state of emergency and restricting movement during the Ebola outbreak.
He said his government did at the time what it could do and did not have the knowledge to fight the disease.
He said his government had to put up with the delays because international organisations such as the WHO “were the experts”.
Neighbouring Liberia was declared Ebola-free in September following 4,800 deaths there.
A handful of cases are still being reported in neighbouring Guinea. Sierra Leone has said it will take heightened security and health screening measures at their shared border.