Rialto (often The Rialto, or Rialto Towers) is a skyscraper located at 525 Collins Street, in the western side of the central business district of Melbourne, Victoria, Australia. It is the second-tallest reinforced concrete building and the tallest office building in the Southern Hemisphere, when measured to its roof. The Rialto featured Melbourne's first skyscraper public observation deck which operated between 1994 and 31 December 2009.