President Julius Maada Bio announced the emergency on Thursday amid a public outcry over the issue, triggered by a series of high-profile attacks.
Mr Bio said attacks on minors – which account for a third of all cases – would be punished with a life sentence.
Activists say many assaults are not punished under current laws.
More than 8,500 cases were recorded last year – a rise of nearly 4,000 on the figure from the previous year – in a country of 7.5 million people.
The factors behind the spike are not yet clear.
Mr Bio declared the emergency at State House in Freetown after hearing the testimony of an Ebola survivor who had repeatedly been raped.
He also announced the formation of a dedicated police division to investigate reports of sexual violence, as well as a special magistrates’ court that would fast-track cases.
The BBC’s Umaru Fofana in Freetown says the declaration of an emergency will mean state resources are more readily diverted to tackling sexual violence.
He added that the move allows the president to bypass parliament, which would normally be required to approve changes to the law.
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