However Portuguese football federation (FPF) Gilberto Madail admitted they were only "moderately optimistic" of winning the right to stage the 2018 showpiece.
Heading into Thursday's vote by FIFA's executive committee -- which will also name the hosts of the 2022 World Cup -- England and Russia lead the race to host 2018.
Also bidding for the event is Netherlands-Belgium.
The race to win the right to host both the 2018 and the 2022 editions has however been dogged by controversy.
A Sunday Times investigation snared two FIFA members apparently offering to sell their votes in exchange for cash. Both officials were later suspended by FIFA and will not take part in the vote.
On Monday, BBC documentary Panorama accused three more FIFA executive committee members of involvement in a decade-old corruption scandal.
A fourth FIFA member, Trinidadian official Jack Warner, seen as a key figure for English hopes, was accused by the programme of trying to sell World Cup tickets on the black market.
Madail said Wednesday that the Iberian bid was "untainted".
"We have nothing to fear and we haven't made any deals or alliances," he added. "We're moderately optimistic. I think we have as much chance as the other (bids). Our bid has been well thought out, and well prepared."
Both nations have previous experience of staging major tournaments with Portugal hosting Euro 2004 while Spain were the hosts for the 1982 World Cup and the 1992 Olympics in Barcelona.