Polling stations will open at 09:00 a.m. local time (0800 GMT) on Sunday and will close at 8:00 p.m. local time (1900 GMT), with some 36.5 million people eligible to vote, Reurters reported.
About one in three of the 36.5 million eligible voters are still undecided.
Opinion polls show the ruling conservative People's Party (PP) of Prime Minister Mariano Rajoy will win the vote but fall well short of an absolute majority.
The Socialists are expected to come second with anti-austerity party Podemos ("We Can") and a second major newcomer, liberal Ciudadanos ("Citizens"), vying for the third place which would make them kingmakers in post-election talks.
That prediction makes any of three outcomes possible - either a right-wing or left-wing coalition government or a minority administration.
Rajoy said on Wednesday he would consider a cross-party pact to ensure a stable administration over the scheduled four-year term, but all the other main parties have come out against joining the PP in a coalition.