His absence has thrown Africa’s most populous nation – which is the No. 3 crude oil supplier to the US – into a political crisis. (READ OUR STORY: Can Nigeria, still without its president, avoid a political crisis?)
Power was earlier this month passed to his deputy Goodluck Jonathan after weeks of jockeying for power among government officials, opposition parties, and a populace increasingly concerned over who was running their country.
Who's in charge?
Now that Yar’Adua has returned, it is not clear who will be in charge of the nation and its 150 million people, split almost equally between Muslims and Christians.
For the time being, Mr. Jonathan "will continue to oversee the affairs of state," according to a statement from presidential spokesman Olusegun Adeniyi. But the statement offered no other details about Yar'Adua's health and how long he might be unable to lead.
Jonathan has already sacked a number of high-ranking officials seen as allies to Mr Yar’Adua, reshuffled ministries, and moved to stamp his authority on plans for a peace amnesty for rebels in the oil-rich Niger Delta.
There are concerns that the Jonathan's actions may make it difficult for his boss to regain the levers of power.
Washington welcomed news of Yar’Adua’s return, but raised concerns that it may have been too early if the president is not yet ready to carry out the affairs of state.
"We hope that President Yar'Adua's return to Nigeria is not an effort by his senior advisers to upset Nigeria's stability and create renewed uncertainty in the democratic process," Assistant Secretary of State Johnnie Carson said in a statement.
Added to the mix is an unspoken agreement in elite Nigerian politics that the presidency swaps between a southerner and a northerner every eight years.
This gentlemen's agreement is designed to soften tensions between the two halves of the country. Yar’Adua is from Kaduna in the mainly Muslim north. His predecessor, Olusegun Obasanjo, was a southerner.
Jonathan is from the southern oil-producing Bayelsa state, and if he holds on to power, there may be anger from the northern elite at being cheated of their eight years in control.