Gyanendra has only months left as king
The Maoists left the government in September, vowing not to return unless the monarchy was scrapped. They ended a decade-long insurgency last year.
Nepal will be declared a republic after elections in April next year.
King Gyanendra, whose dynasty dates back to 1769, lost popular support when he sacked the government in 2005 and assumed absolute power.
Nepal home minister
The decision to make Nepal a "federal democratic republican state" was taken by an overwhelming majority - 270 MPs out of 371 voted to abolish the monarchy, with only three against.
The main political parties had originally agreed to leave the question of whether Nepal should become a republic to the constituent assembly being elected in April.
But the Maoists wanted the decision taken at once - hence the agreement reached by the main political parties earlier this week. It will allow the Maoists to re-join the administration.