|Full name||Mario Sebastián Viera Galaín|
|Date of birth||7 March 1983|
|Place of birth||Florida, Uruguay|
|Height||1.85 m (6 ft 1 in)|
|* Senior club appearances and goals counted for the domestic league only and correct as of 17 March 2019|
Born in Florida, Florida Department, Viera started his professional career with Club Nacional de Football. After one sole season he was expected to move to Premier League club Arsenal in August 2005, but its manager Arsène Wenger withdrew from the deal when the player did not pass his medical.
Eventually, Viera signed with Villarreal CF in Spain, moving alongside Mariano Barbosa (also a goalkeeper) on a four-year contract. He would be an undisputed starter in the following two seasons, although he missed both legs of the 2005–06 UEFA Champions League semi-finals against Arsenal.
Viera eventually lost his job midway through the 2007–08 campaign to newly signed Diego López, and never regained it again – no La Liga games in 2008–09 – being released in July 2009. On 11 November he signed a one-and-a-half-year contract with Greek side Athlitiki Enosi Larissa FC, being made eligible to play however only in January 2010; his contract was terminated late into that year, and he joined Atlético Junior shortly after, remainining in Colombia for several years.
Viera made his debut for Uruguay on 18 July 2004 in a Copa América quarter-final match against Paraguay, having been called as a late replacement for injured starter Fabián Carini. He also featured in the penalty shootout loss to eventual winners Brazil.
Viera's father, also named Mario, was also a professional footballer – and a goalkeeper.
- Atlético Junior
- Henry talks to start soon; Sky Sports, August 2005
- Viera switch ends Villarreal search; UEFA.com, 29 August 2005
- Sebastián Viera habla de Diego López, arquero del polémico Real Madrid (Sebastián Viera talks about Diego López, goalie of controversial Real Madrid); El Tiempo, 6 May 2013 (in Spanish)
- Στα βυσσινί ο Βιέρα (Viera in crimson); AEL 1964, 11 November 2009 (in Greek)
- Πήρε... πόδι ο Βιέρα (Viera... got the boot); Sport Dog, 29 December 2010 (in Greek)
- Copa América 2004; at RSSSF
- Arquero uruguayo del Junior asegura que la confianza en Dios le permitió tapar un penal (Uruguayan Junior goalie assures confidence in God enabled him to save a penalty); El País, 22 December 2011 (in Spanish)