The 58-year-old leader, who has long suffered from kidney ailments, has not made a public appearance since he left for treatment in Saudi Arabia at the end of November. He was flown back to Nigeria two months ago but remains too frail to govern.
Goodluck Jonathan, his deputy, has taken over his executive powers as acting president in his absence.
Yar'Adua's secretive return in the middle of the night raised fears that his inner circle of aides, led by his wife Turai, would fight to maintain their influence over Africa's most populous nation and seek to undermine Jonathan.
|Jonathan has taken over as the acting president in view of Yar'Adua's ill-health [AFP]
The country had been plunged into a near constitutional crisis, with protests held in the capital demanding Yar'Adua's resignation while he was away for treatment.
Jonathan sacked his entire cabinet in March only a month after parliament empowered him to run the country.
The senate on Wednesday confirmed 38 new ministers proposed by Jonathan for his new cabinet.
Jonathan, from the southern Niger Delta, is unlikely to run in elections in April next year because of an unwritten agreement that power rotates between the north and south.
Yar'Adua is a Muslim from the north and his predecessor, Olusegun Obasanjo, was a Christian from the south.
A power struggle could paralyse the Nigerian government and threaten an amnesty programme in the oil-producing Niger Delta, jeopardising reforms in sectors from banking to oil and gas.