William Iain Henderson (born 21 February 1992) is a professional rugby union player who plays as a lock for United Rugby Championship club Ulster and the Ireland national team, and has been selected for two tours with the British & Irish Lions.

[8] He made his debut for the senior Ulster team in April 2012,[4] and signed his first professional contract in October 2012.

[15] With Ireland, he won the Six Nations Championship in 2014 and 2015, the Grand Slam in 2018, and the Triple Crown in 2022, and played in the Rugby World Cup in 2015 and 2019.

[16] Henderson's central contract with the IRFU was due to expire after the 2023 Rugby World Cup, and there was speculation that he might move abroad.

[17] He was called up to Ireland's preliminary 42-man squad ahead of the 2023 Rugby World Cup,[18] and captained the side in the warm-up game against Italy on 5 August 2023.