"The World Health Organisation joins the government of Sierra Leone in marking the end of the recent flare-up of Ebola virus disease in the country," a statement said.
The global health body made the announcement after 42 days -- or two Ebola incubation cycles -- since Sierra Leone's last Ebola patient tested negative for the second time.
WHO was however careful to warn that a recurrence of cases of the deadly tropical disease remained possible in the three worst affected countries.
Sierra Leone was previously declared free of Ebola transmission on November 7 last year and Guinea on December 29 while Liberia followed on January 14 -- but only after the virus killed more than 11,300 people across the region.