The president's victory over his closest challenger, Atiku Abubakar, a former vice president, means he'll extend his stay in the Presidential Villa after his first victory in 2015.
While delivering his acceptance speech in Abuja moments later, the president said troublemakers in a handful of states attempted to disrupt the elections, leading to the sad loss of lives.
INEC was forced to postpone elections in certain communities in Lagos, Anambra and Rivers due to violence that made voting impossible. At least six people were killed by soldiers in election-related violence in Rivers State, while a young voter was also killed in Kogi.
Yakubu also revealed that an ad hoc staff of INEC was killed in Rivers State.
During his speech on Wednesday, the president assured Nigerians that security agencies will bring to justice all those arrested in the process.
He said, "I am very sad at the grievous loss of lives during these elections. Security agencies will step up their efforts to protect voters in the forthcoming State elections.
"I would like to commend our security agencies, as severely overstretched as they are, for handling the difficult task of securing the country during the election period."
The president also promised Nigerians he'll further improve on the three focal points of his administration to improve the economy, secure the country and fight corruption.
"The new administration will intensify its efforts in security, restructuring the economy and fighting corruption. We have laid down the foundation and we are committed to seeing matters to the end.
"We will strive to strengthen our unity and inclusiveness so that no section or group will feel left behind or left out," he said.
Buhari beats Atiku with 3.9m victory margin
Buhari won 15,191,847 of the total votes cast while Atiku gained 11,262,978 votes, an astonishing victory margin of 3,928,869 votes.
The president won in 19 states including Ekiti (219,231), Osun (347,634), Kwara (308,984), Nasarawa (289,903), Kogi (285,894), Gombe (402,961), Yobe (497,914), Niger (612,371), Jigawa (794,738), and Kaduna (993,445).
Other states where he won are Bauchi (798,428), Lagos (580,825), Ogun (281,762), Kano (1,464,768), Katsina (1,232,133), Borno (836,496), Sokoto (490,333), Kebbi (581,552), and Zamfara (438,682).
Atiku won in 18 states including the FCT (259,997), Ondo (275,901), Abia (219,698), Enugu (355,553), Ebonyi (258,573), Anambra (524,738), Oyo (366,690), Adamawa (410,266), Edo (275,691), Benue (275,691).
Other states where he won are Imo (334,923), Plateau (548,665), Taraba (374,743), Cross River (295,737), Akwa-Ibom (395,832), Delta (594,068), Bayelsa (197,933) and Rivers (473,971).
Buhari won his first term victory in 2015 after he beat then-incumbent, Goodluck Jonathan, by 2,571,759 votes as he won 15,424,921 votes over his opponent's 12,853,162 votes.
Atiku has not yet called President Buhari to concede defeat like Jonathan did when he was defeated by the former military Head of State in 2015.