Diego da Silva Costa (Spanish: [ˈdjeɣo ða ˈsilβa ˈkosta], Brazilian Portuguese: [dʒiˈeɡu dɐ ˈsiwvɐ ˈkɔstɐ]; born 7 October 1988) is a professional footballer who plays as a striker. Born in Brazil, he represented the Spain national team.

Costa began his career in his native Brazil before joining Braga in Portugal in 2006, aged 17. He never played for the club but spent time on loan at Penafiel, and signed with La Liga club Atlético Madrid the following year. Over the next two seasons, he had loan periods with Braga, Celta Vigo, and Albacete. His form earned him a move to fellow La Liga club Real Valladolid in 2009, where he spent one season, finishing as their top goalscorer, before returning to Atlético Madrid. Costa struggled to maintain a regular starting role with Atlético, and spent more time on loan, this time at Rayo Vallecano, where he finished as the club's highest scorer that season.

In 2011, Costa returned to Atlético with a greater role. He blossomed as a goalscorer, and helped the team win a La Liga title, a Copa del Rey title, and a UEFA Super Cup, as well as reaching the 2014 UEFA Champions League final. In 2014, he was signed by Premier League club Chelsea in a deal worth €35 million (£32 million). In London, Costa won three trophies, including two Premier League titles and a League Cup. In 2018, following a rift with head coach Antonio Conte, Costa returned to Atlético Madrid in a club record transfer worth an initial €56 million, where he won a UEFA Europa League title and another UEFA Super Cup as well as a second La Liga title in 2021.

Costa is a dual citizen of Brazil and Spain. He played twice for Brazil in 2013, before declaring his desire to represent Spain, having been granted Spanish citizenship in September that year. He made his debut for Spain in March 2014 and has since won 24 caps and scored 10 goals, and has represented them at the 2014 and 2018 FIFA World Cups.

Known for his fiery temperament, Costa has been criticised and punished for several confrontations with opponents.

Costa was born in Lagarto, Sergipe, Brazil, to parents José de Jesus and Josileide. His father named him in honour of Argentine footballer Diego Maradona despite the rivalry between the two nations, and he has an elder brother named Jair after Brazilian player Jairzinho. Despite regularly playing street football, Costa did not believe as a child that he would turn professional, in part due to the remote location of his hometown. He has since set up a football academy in his hometown, where he pays all the costs. Costa is a fan of Palmeiras.

Costa trialled unsuccessfully at his hometown team Atlético Clube Lagartense. At age 15, he left Sergipe and moved to São Paulo, to work in the store of his uncle Jarminho. Although he was never a professional, Jarminho had connections in football and recommended his nephew to Barcelona Esportivo Capela, a team from the south of the city set up as an alternative to drugs and gangs for youth of the favelas. Before joining this team, he had never been coached in football. He turned professional at the club, earning around £100 per month, and competed in the under-18 Taça de São Paulo despite a four-month ban for slapping an opponent and dissent towards the referee. Although he was sent off in the first game of the tournament, he attracted the attention of renowned Portuguese agent Jorge Mendes, who offered him a contract at Braga. Costa's father was apprehensive of sending his son to Europe, and suggested he instead sign for nearby Associação Desportiva São Caetano, but he was adamant that he would take the opportunity.

Jair played on the same team as Diego, and was a slimmer, more technically able player, but had less focus; the two were often not fielded at the same time in order to prevent arguments. He never turned professional, but had a three-month trial at Basque club Salvatierra.