From 2018, Rees-Zammit played as a winger for Gloucester in Premiership Rugby, between 2020 and 2023 at international level for Wales, and in 2021 for the British & Irish Lions.

He subsequently signed for the Jacksonville Jaguars as part of their practice squad as a wide receiver for 2024 and was made a free agent after the end of the season, before he was resigned as a full active roster member in February 2025.

Louis Rees-Zammit was born in Penarth, Vale of Glamorgan, Wales;[1] his paternal grandfather had emigrated from Malta.

[9] In 2021, due to Rees-Zammit's success on the rugby field, he was chosen as the official patron for the British charity Sporting Minds UK, with which he had worked since 2019.

In 2017, Rees-Zammit transferred to Hartpury College, the academy of Gloucester Rugby, in the AASE Championship, winning consecutive trophies.

As part of the Gloucester youth team, Rees-Zammit played in two consecutive Premiership Rugby U18 Academy League finals.

[11][12] Louis Rees-Zammit debuted in the Gloucester senior team in the 2019–2020 season, becoming the club's youngest ever English Premiership player at 18 years and 70 days.

Gloucester reached the quarter-finals of the 2022 European Challenge Cup, with Rees-Zammit scoring a try during the match, which they lost to Saracens.

[26] In this season, Rees-Zammit played 13 league games, started in 12 of those, and scored four tries, as well as one try in a single appearance in the European Rugby Champions Cup.

[27] In January 2024, Rees-Zammit decided to leave Gloucester, by which time he had made 77 club appearances and scored 210 points.

[11] He received his first full call-up to the senior Wales squad under coach Wayne Pivac on 15 January 2020 as a potential replacement for the 2020 Six Nations Championship.

He was selected for Wales' squad for the 2023 Rugby World Cup competition in France, scoring five tries in five appearances, although he started only two of those matches.

[42] His inclusion at the age of 20 years and 93 days made him the youngest player to be selected since the 1959 Lions tour to Australia and New Zealand.

[44] He was selected to play against Sharks in the following game, again scoring a try as the Lions won the match, finishing 54–7.

Rees-Zammit has compared his playing style to that of the NFL players Deebo Samuel and Christian McCaffrey of the San Francisco 49ers.

[6][49] Prior to Rees-Zammit's professional NFL debut, he was already considered one of the league's fastest athletes alongside John Ross and Chris Johnson.