"I commend yesterday's peace accord in Nigeria," Biden said in a statement, referring to a pledge signed by all candidates on Wednesday to seek redress through the courts for any grievances.
"By signing this pledge, the parties and candidates have committed to accept the results of the election, as announced by the Independent National Electoral Commission (INEC), and to support a peaceful transition of power," he said.
More than 93 million Nigerians, out of a total population of over 200 million, are registered to vote in Saturday's presidential and parliamentary election, seen as the most wide open since the country returned to democracy in 1999.
"All Nigerians deserve this chance to choose their future — freely and fairly," Biden said.
"While the United States does not support any single candidate or party, we strongly support a peaceful and transparent process that reflects the will of the people of Nigeria."
Nigeria has a long history of electoral fraud and violence, though its elections have got gradually cleaner in the near quarter century since it moved away from army rule. INEC says it has taken measures to ensure voting will be free and fair.