William John Paul Gallagher (born 21 September 1972) is an English singer and songwriter who is the lead singer and co-founder of the rock band Oasis. Gallagher fronted Beady Eye, an offshoot of Oasis, between 2009 and 2014, before starting a solo career in 2017. One of the most recognisable figures in British rock music, he is noted for his distinctive vocal style and outspoken personality.

Gallagher was interested in joining a band called the Rain. After mutual agreement, the band underwent a name change to Oasis, and his older brother Noel was initially approached to be their manager. Noel was subsequently extended an invitation to join the band as their lead guitarist and songwriter, which he accepted. The band's debut album, Definitely Maybe (1994), was a critical and commercial success amidst the emergence of Britpop. Their second album, (What's the Story) Morning Glory? (1995), reached the top of the album charts in many countries, and their third studio album, Be Here Now (1997), became the fastest-selling album in UK chart history. Britpop eventually declined in popularity, and Oasis failed to revive it; however, all of their subsequent albums topped the UK charts, and they continued to tour, playing gigs to 1,000,000+ people worldwide, but particularly in Europe, and South America. In August 2009, following Noel's departure from Oasis, Gallagher and the remaining band members would go on to form Beady Eye, with whom he released two studio albums before they disbanded in 2014.

In 2017, Gallagher began his solo career with the release of his debut solo album, As You Were (2017), which proved to be a critical and commercial success. It topped the UK Albums Chart and was the ninth fastest-selling debut album of the 2010s in the UK, with over 103,000 units sold in its first week. In 2018, the album was certified platinum with over 300,000 units sold in the UK. His second album, Why Me? Why Not, received mostly positive reviews and topped the UK charts upon its release in September 2019. This made it his tenth chart-topping album including eight with Oasis, and it also became the fastest-selling vinyl of 2019. In March 2010, he was voted the greatest frontman of all time in a reader poll by Q magazine. In 2019, he received the MTV Europe Music Award for "Rock Icon". Gallagher's third album, C'mon You Know, was released in 2022 and became his fourth solo album to debut at number one on the UK Albums Chart. In early 2024, Gallagher collaborated with ex-Stone Roses musician John Squire on the studio album Liam Gallagher John Squire, which was released that March and continued Gallagher's streak of debuting atop the British charts.

Oasis's time was marked by turbulence, especially during the peak of Britpop, during which Gallagher was involved in several disputes with Noel. Their conflicts and wild lifestyles regularly made tabloid headlines. The band had a rivalry with fellow Britpop band Blur.