Generally not required for stays of up to 90 days per 180 days (visas are not required at all for members of EU or Schengen countries). Some nationalities need a Schengen visa.
Spain is one of 26 member countries of the Schengen Convention, under which 22 EU countries (all but Bulgaria, Cyprus, Ireland, Romania and the UK) plus Iceland, Norway, Liechtenstein and Switzerland have abolished checks at common borders.

The visa situation for entering Spain is as follows:

Citizens or residents of EU & Schengen countries No visa required.

Citizens or residents of Australia, Canada, Israel, Japan, New Zealand & the USA No visa required for tourist visits of up to 90 days out of every 180 days.

Other countries Check with a Spanish embassy or consulate.

To work or study in Spain A special visa may be required – contact a Spanish embassy or consulate before travel.