, also known mononymously as Casemiro [ka.zeˈmi.ɾu], is a Brazilian professional footballer who plays as a defensive midfielder for Premier League club Manchester United and the Brazil national team.

Casemiro began his career with São Paulo and progressed through the ranks. He scored 11 goals in 111 games as a midfielder for the club. His defensive abilities caught the attention of Real Madrid, and he joined the club on loan in January 2013. He played 15 games for Real Madrid Castilla during his loan spell. Afterward, he was signed permanently by Real Madrid for a fee of £5.1 million, becoming a part of their first-team squad at the Santiago Bernabéu.

After joining Real Madrid, Casemiro initially struggled to break into the first team. He was loaned to FC Porto for €15 million in order to gain more playing time. During his time at Porto, Casemiro performed impressively and helped the team reach the Champions League quarterfinals. Real Madrid decided to reacquire him for €7.00m in the summer transfer window of 2015. After returning to the club, Casemiro played a crucial role in Real Madrid's success, becoming one of the best defensive midfielders in the world, and winning numerous trophies including three La Liga titles, three Supercopa de España, three FIFA Club World Cups, three UEFA Super Cups, one Copa del Rey and five UEFA Champions Leagues. In 2022, he left Real Madrid and joined Manchester United for a transfer worth a reported €70.65m, winning the EFL Cup in his first season.

A full international since 2011, Casemiro was in Brazil's squad at the 2018 and 2022 FIFA World Cups as well as four Copa América tournaments, winning the 2019 Copa América and finishing as a runner-up in 2021.