Marcelo Vieira da Silva Júnior (born 12 May 1988), known as Marcelo, is a Brazilian professional footballer who plays as a left-back for Super League Greece club Olympiacos. Known for his offensive capabilities, trickery, and technical qualities, Marcelo is often regarded as one of the greatest left-backs of all time. He has spent most of his career with Spanish club Real Madrid and is the club's most decorated player, with 25 trophies won.

Starting his professional career in 2005, Marcelo won a Campeonato Carioca with Fluminense in 2006, during which he was named to the Brasileirão Team of the Season, at age 18. By the end of the year, he joined Real Madrid for an $8 million fee. In Madrid, he went on to be a mainstay and has won five UEFA Champions League titles and six La Liga titles, among other honours. In 2021, Marcelo became the captain of Real Madrid from the start of the 2021–22 season, making him the first foreign captain since 1904. He has been named to the FIFPro World XI six times, the UEFA Team of the Year three times, and La Liga's Team of the Season in 2016. He was also named to France Football's "Team of the Decade (2010–2019)".

Having debuted for Brazil in 2006, Marcelo was called up for the 2008 Olympic squad, where he won a bronze medal. Four years later, he was part of Brazil's 2012 Olympic team as one of the three over-age players, winning a silver medal. Marcelo was selected to play in the 2013 FIFA Confederations Cup and started all five matches, as Brazil won 3–0 against Spain in the final. He was part of Brazil's squad at the 2014 FIFA World Cup which was his first World Cup. Brazil finished fourth and he was named to the tournament's Dream Team. In May 2018, he was named to the final squad for the 2018 FIFA World Cup where he again was named to the tournament's Dream Team.