Koroma swept to a second term with 58 percent of votes in a poll that observers praised as peaceful and transparent, triumphing over his main rival Julius Maada Bio who trailed with 37.4 percent of votes.
By scoring more than 55 percent of the vote the incumbent managed to avoid a second round of voting.
Koroma was sworn-in immediately the results were announced. He called on all Sierra Leoneans, including Bio's opposition Sierra Leone People's Party (SLPP), to unite in moving the country forward.
"The job at hand requires the goodwill and positive energy of the membership of all political parties," he said.
"We will focus on creating jobs for the youths, and on training our youths to seize the immense employment opportunities we are creating in the construction, mining, agriculture and other sectors," Koroma said.
"We will continue with our infrastructural development programmes; we will continue to attract investment; we will continue to fight corruption," said Koroma.
During his campaign he was heavily criticised for his failure to crack down on graft.
The crucial test for the west African nation will be whether the ex-military leader Bio accepts the outcome. During the counting of the votes, he alleged fraud and vowed that he would not let his supporters be cheated.
Announcing the result Friday evening, Thorpe said: "Any citizen of Sierra Leone may challenge the validity of the election of the president by petition to the Supreme Court of Sierra Leone, within seven days after the declaration of the presidential results."
The country still bears the scars of a brutal 11-year civil conflict during which rebels hacked off the limbs of civilians during a campaign of terror. They bankrolled their army with the sale of so-called "blood diamonds".
Now accustomed to peace, Sierra Leoneans who are still amongst the world's poorest people, voted for greater development as the country expects a lucrative windfall from its rich mineral resources.
Koroma's government will have the stewardship of a booming mining industry, notably iron-ore, and possible oil production.
Sierra Leone is rich in mineral resources and massive iron-ore stores are expected to add 21 percent growth this year to its $2.2-billion (1.7-billion-euro) gross domestic product, the International Monetary Fund estimates.
If well-managed, these resources could change the fortunes of a nation which has one of Africa's lowest life expectancies at 47 years, according to the World Bank, and highest rates of maternal mortality. Youth unemployment levels hover at 60 percent.
"We will construct roads in every region, continue to bring electricity to every district; develop agriculture in every chiefdom and provide free healthcare for the mothers and children of every village," Koroma said Friday.
National Elections Commission chief Christiana Thorpe said Koroma had won 1,314,881 votes to Bio's 837,517 in an election which had a massive 87.3 percent voter turnout.
In third place was Charles Margai of the People's Movement for Democratic Change (PMDC), who won only 1.3 percent of the vote.
Sierra Leone's third election since the war ended in 2002 was widely praised as peaceful, transparent and well-conducted by observers, who wanted to see if the country had cemented its status as a stable democracy.
Koroma was first elected in 2007 in an election that saw pockets of violence, but eventually led to a peaceful transfer of power from the opposition SLPP to his All People's Congress (APC).
No results were announced for parliamentary and local elections, which were held simultaneously, the first polls wholly organised by government since the end of the conflict.
