Joanna Muir (born 30 August 1994) is a British modern pentathlete, who was part of the British squad that won the team event at the 2019 European Modern Pentathlon Championships, and came second in the women's relay event at the 2016 World Modern Pentathlon Championships. She came second at the opening event of the 2021 Modern Pentathlon World Cup. Muir competed in the modern pentathlon event at the delayed 2020 Summer Olympics.