35-year-old Adama Sankoh was discharged from a treatment centre in the northern Bombali district on Monday morning.
The country hasn't reported a new infection for more than two weeks, according the the National Ebola Response Centre (NERC).
However, a small number of cases are still being reported in neighbouring Guinea.
The milestone comes 15 months after the outbreak was declared in Sierra Leone. Almost 4,000 people have died of Ebola in that country since then.
The country's President Ernest Bai Koroma attended the celebrations at the clinic run by International Medical Corps.
Vanessa Wolfman, the Medical Director at the facility, said: "Today is a day of hope. People hope this is the beginning of the end. We have fought for so long."
But the celebrations are tempered with caution. The country still has 28 people in quarantine. They will continue to be monitored until the end of the week in case they develop symptoms.
OB Sisay, Director of the situation room of NERC, said: "We might have hidden cases, so we have to continue to be vigilant, continue our surveillance, maintain our discipline of hand-washing and temperature checks, screening and avoid over-crowding."
The outbreak won't be declared over until 42 days after the last Ebola patient either dies or is discharged. That 42-day limit is twice the incubation period of the virus.
While there have been no new cases in Sierra Leone for the past 17 days, according to NERC, neighbouring Guinea reported three new infections in the week up to 19th August.
Mr Sisay said: "We are very worried [about Guinea]. We are fervently hoping that our cousins there will make the same progress as we have... Guinea's problem is our problem."
Not over yet
Hundreds of millions of dollars have been poured into dealing with the Ebola outbreak.
At the height of the epidemic, dozens of centres opened in Sierra Leone where Ebola patients could be isolated and treated. Now there are 10.
The country's Ministry of Health is in the process of deciding how many treatment centres it will keep open after the epidemic. The already weak health system was decimated by the outbreak.
But Vanessa Wolfman from the International Medical Corps said it had no immediate plans to close the British-funded facility, including a diagnostics lab.
Symptoms for malaria, a disease that is widespread in Sierra Leone, are similar to those of Ebola, so medics are still seeing suspected cases.
Dr Wolfman said: "As we see the tail end of this we still have safe places for sick people to be referred."
"We can get people in, isolate them from other community members, do rapid testing and get them back home as soon as possible," she added.
The NERC said it will continue with heightened surveillance, and specific anti-Ebola safe burial practices, for three months after the outbreak is officially declared over.