The impoverished country, along with neighbouring Guinea and Liberia, is struggling to contain an epidemic that has infected 1,200 people and left 672 dead across the region since the start of the year.
"Extraordinary challenges require extraordinary measures. The Ebola virus disease poses an extraordinary challenge to our nation," Koroma said in a televised address to the nation.
"Consequently... I hereby proclaim a state of public emergency to enable us to take a more robust approach to deal with the Ebola outbreak."
Koroma said he had cancelled a trip to a summit of around 50 African leaders in Washington DC next week.
He announced however that he would travel to neighbouring Guinea for a regional summit on the crisis gathering the heads of Sierra Leone, Liberia, Guinea and Ivory Coast.
Koroma announced a raft of measures as part of the state of emergency, including quarantining Ebola-hit areas and deploying security forces to protect medical workers.
He banned all public meetings not related to Ebola and cancelled foreign trips by ministers and other government officials, exempting only "absolutely essential engagements".
The president said the measures would be in place initially for 60 to 90 days, and then be reassessed.
Sierra Leone, which has seen 224 deaths, was also preparing Thursday to bury Omar Khan, a "national hero" who saved the lives of more than 100 Ebola patients before succumbing to the tropical bug.
'Very serious threat'
Fears that the outbreak could spread to other continents have been growing with European and Asian countries on alert.
Leading medical charity Doctors Without Borders warned the crisis would only get worse and said there was no overarching strategy to handle the world's worst-ever outbreak of the disease.
Sierra Leone's announcement comes a day after Liberia, which has seen 129 deaths, said it was shutting all schools and placing "non-essential" government workers on 30 days' leave.
US Christian charity Samaritan's Purse said it was temporarily withdrawing its non-essential staff from Liberia, citing regional "instability and ongoing security issues".
Hong Kong announced quarantine measures for suspected cases, although one woman arriving from Africa with possible symptoms tested negative, while the EU said it was ready to deal with the threat.
The International Civil Aviation Organization (ICAO) has held talks with global health officials on potential measures to halt the spread of the disease.
In Britain, where one person has tested negative for the disease, Foreign Secretary Philip Hammond said it was regarded as "a very serious threat".
The US Peace Corps announced Wednesday it was pulling hundreds of volunteers from Guinea, Liberia and Sierra Leone.
The European Union is equipped and ready to treat victims should the deadly virus be found in its 28 member states, an EU source said in Brussels.
Medical staff 'swamped' -
In Hong Kong, a densely populated city previously scarred by disease outbreaks such as the 2003 SARS epidemic, health officials confirmed they would quarantine as a precautionary measure any visitors from Guinea, Sierra Leone and Liberia who showed fever symptoms.
One woman arriving in the southern Chinese city from Africa, who showed symptoms including fever and vomiting, has tested negative for Ebola.
Australia, which has already warned against travel to west Africa's Ebola-hit countries, said it was well prepared in the unlikely event that the virus should reach its shores.
Meanwhile, Thai health authorities said they had ordered all hospitals to monitor patients for any symptoms, particularly nationals or foreign tourists who had been in the outbreak area.
A British doctor volunteering in Sierra Leone treating Ebola patients told Metro newspaper that medical staff were swamped.
"The main challenge here, though, is that the health authorities just don't have the infrastructure to cope. They're overwhelmed," Benjamin Black said.
Togo-based pan-African airline ASKY, which serves 20 destinations, has halted all flights to and from Liberia and Sierra Leone following the death of a passenger from the virus.
The 40-year-old man, who travelled from Liberia, died in Lagos on Friday in Nigeria's first confirmed death from Ebola.
The virus crossing borders for the first time by plane could lead to new flight restrictions aimed at containing outbreaks, the world aviation agency said.
Japan's health ministry said it has sent out an alert to hospitals and health organisations to be on the lookout for people with signs of the virus.
"In addition to the ordinary measures we already have in place, including thermographies to check on travellers' body temperatures (at the airport), the government is drawing attention to the concerned authorities," a ministry spokesman said.