UN boss António Guterres has a laid a wreath at the global body’s main office in the Nigerian capital, Abuja, to honour those who died when a suicide bomber targeted the building in 2011.
At least 24 people were killed and dozens of others wounded in the car bomb attack by the Islamist militant group Boko Haram, drawing international outrage and condemnation.
The victims included many UN staff.
The Nigerian government later rebuilt the devastated building, known as UN House - where all UN agencies working in Nigeria have their offices.
The UN chief held a closed-door meeting with UN staff and is now meeting Nigerian President Muhammadu Buhari and other senior government officials at the presidential palace.
On Tuesday, Mr Guterres was in the north-eastern state of Borno, where UN agencies and humanitarian groups have been helping thousands of families who have fled attacks by Boko Haram insurgents.
He also met former members of the extremist group at a de-radicalisation camp, where he called for more investment to improve people’s lives as a way to tackle the “enormous” challenges facing the region.
The UN says the Boko Haram insurgency, which began in 2009, has caused the death of nearly 350,000 people and pushed millions more from their homes in Nigeria and the neighbouring countries of Niger, Cameroon and Chad.