Paul Gallen is a former professional rugby league footballer who played as a lock, prop and second-row forward and captained the Cronulla-Sutherland Sharks in the NRL to their maiden NRL Premiership in 2016. He is a former captain and representative of the New South Wales State of Origin team. He has also been the vice captain of Australia and played his whole NRL career with the Sharks. Gallen was the NRL's oldest player playing in 2018. He shares the Australian first grade record for most seasons played in the NRL with Cameron Smith at 19 seasons.

Paul Gallen played his junior rugby league for the Wentworthville Magpies and was in the Parramatta Eels junior system playing the S. G. Ball Cup before being overlooked by the Eels. Gallen later signed with the Cronulla-Sutherland Sharks. In June 2001, Gallen made his NRL debut for the Cronulla-Sutherland Sharks against the Parramatta Eels in round 15 at Toyota Park. Cronulla lost the match 36 to 6. He made one more appearance that season in the final round, where the Sharks were victorious over the Brisbane Broncos.

Gallen scored his first ever try at first-grade level during round 14 of the 2002 season - against the Canberra Raiders. Gallen made 21 appearances in his second season at first-grade level for the Cronulla-Sutherland Sharks, scoring one try during the season. During the round 11 match (in 2003) against the Newcastle Knights, Gallen was sent off by referee Paul Simpkins after a high shot on Sean Rudder.

In the opening round of the year, against the Wests Tigers, Gallen was involved in a fight which saw him suspended for two weeks after the NRL Judiciary found him guilty of a contrary conduct charge. In round 11 against the Manly-Warringah Sea Eagles, Gallen scored an impressive two tries in his sides 30 points to 28 victory. In the 2004 season, Gallen made a total of 19 appearances for the club and scoring five tries.

In the 2005 NRL season, Gallen played a total of 25 games in first-grade for Cronulla-Sutherland and in the process scored six tries. Gallen ran 3,920 metres with the ball in 2005, more than any other player in the competition. In 2006 Gallen won his first ever representative jersey when he was selected to play for City in the annual City v Country match. . In June, Cronulla coach Stuart Raper pleaded with the New South Wales selectors to give Gallen an opportunity to prove his worth at State of Origin level, however he was only selected for the Game III dead rubber.

In 2007 Gallen reportedly turned down a $1.4 million offer from Manly in favour of a $1.1 million deal. Manly confirmed that they were interested in signing him, however Manly denied that they ever made an actual offer for him. Gallen stated that he decided to stay at the Sharks because he was inspired by Nathan Hindmarsh, who had recently signed as a one-club man. Gallen also cited the fact that he wanted to win a premiership with the club because "you'd be remembered for your whole life here in Cronulla if you were part of that first-ever premiership" as well as the fact that his "old man also wanted me to stay at the Sharks."

Gallen was selected for all three games in the State of Origin series in 2008. In the third and decisive game of the series Gallen gave away three penalties and came under heavy criticism from fans for his lack of discipline, in particular for his late hit on Johnathan Thurston. Gallen described the loss as the worst of his career. In 2008, Gallen represented Australia in the centennial test, scoring a try. In August, Gallen was named in Australia's preliminary 46-man Kangaroos squad for the 2008 Rugby League World Cup, and in October 2008 he was selected in the final 24-man Australia squad.

In August 2014, Gallen was one of the Cronulla players who pleaded guilty to having unknowingly used banned peptides, and in exchange was given a reduced, backdated ban from ASADA and Sports Minister Peter Dutton for his part in the 2011 supplements scandal. Prior to pleading guilty, Gallen stated that he and the other players will "be labelled drug cheats if they accept reduced bans". Gallen captained the New South Wales team to victory over Queensland in the 2014 State of Origin series. The win marked the first time that New South Wales had won the series during the current decade and it put an end to Queensland's record of eight series wins in a row.

Gallen captained Cronulla-Sutherland to the 2016 NRL Grand Final. After a closely contested game, the team went on to win the Premiership, defeating the Melbourne Storm 14–12, the Sharks' first premiership in their 49-year history. Gallen himself played a key role, setting up Ben Barba for the first try of the night to put his side ahead. In 2017, Gallen was named in The City side in the last ever City v Country representative match. On 10 September 2017 Cronulla played against North Queensland in the elimination finals game. Cronulla were expected to win comfortably against a depleted NQ side but found themselves in a tight match at 14-14. With under a minute left, Gallen got within inches of the line for the match winning try but lost the ball. In extra time, Michael Morgan kicked a field goal for North Queensland to make it 15-14 which remained the final score and Cronulla were eliminated from the finals.

After Round 3, Gallen became the first player in NRL History to achieve 50,000 running metres. His closest rival is Billy Slater with 39,439 running metres. Gallen made 21 appearances for Cronulla in 2018 as the club reached the preliminary final before losing to Melbourne 22–6. 2019 was the year that Gallen took over from Scott Prince (166 Losses) as the player with the most losses in NRL history. In August 21, 2019, Gallen was voted as one of the best players of the decade in the NRL team of the decade announcement which spanned from the 2010 to 2019 seasons.

Gallen's final game as a player came in the elimination final as Cronulla were defeated by Manly-Warringah 28–16 at Brookvale Oval. Gallen began his amateur boxing career in 2012 in part of a rugby league versus rugby union charity event in Auckland, New Zealand. In 2014, Gallen turned professional after it was confirmed he would fight Australian-based Herman Ene-Purcell. Since 2018, he has appeared on 100% Footy alongside James Bracey and Phil Gould - a rugby league show that airs on Monday nights on Channel Nine. Gallen has appeared sometimes on Sports Sunday and Nine's Rugby League Coverage.