The signing was done at the Presidential Villa in Abuja on Thursday and was confirmed on the official Twitter handle of the presidency.
The signing of the budget comes exactly ten days after both chambers of the National Assembly passed the N13.6 trillion budget for 2021 and three days after it was transmitted to Mr Buhari for assent.
With the president’s assent, the bill has become law and the implementation is expected to commence in January for 2021 fiscal year.
The legislators had approved N13.6 trillion (13,588,027,886,107) against the N13.08 trillion proposed by the president in October.
The lawmakers added N505 billion to the proposed budget.
A breakdown of the approved budget figures shows about N496.5 billion approved for statutory transfers and N3.3 trillion for debt services.
The recurrent expenditure was put at N5.6 trillion with capital expenditure at N4.1 trillion and fiscal deficit at N5.2 trillion (5,196,007,992,292).
On Thursday, the president also signed the 2020 Finance Bill into law.
The News Agency of Nigeria reports that those who witnessed the signing of the budget include Vice President Yemi Osinbajo, Senate President Ahmad Lawan and the Speaker of the House of Representatives, Femi Gbajabiamila.
Others at the Council Chamber were the Secretary to the Government of the Federation, Boss Mustapha, some cabinet ministers as well as a few top government officials.