Arthur Nicolas Theate (born 25 May 2000) is a Belgian professional footballer who plays as a centre-back for Ligue 1 club Rennes and the Belgium national team.

In the summer of 2020, Theate saw no prospects of succeeding at Standard Liege, after which he was offered a trial with Belgian First Division A club Oostende. After a week of practicing with the first team, he was able to convince German coach Alexander Blessin of his abilities. Blessin mainly noticed his explosiveness and power, ideal for his high five-man backline and Gegenpressing tactics. Previously, Theate had not been able to convince Lommel or Lierse Kempenzonen during trials.

In July 2020, he signed a three-year contract with an option with Oostende. Theate made his debut on 10 August 2020 in the Belgian First Division A against Beerschot, featuring in starting line-up. Together with Anton Tanghe he grew into the permanent centre-back duo in the Oostende defence. On 4 October 2020, he scored his first professional goal, heading home a cross against Royal Excel Mouscron putting his team in a good position ahead of their 3–0 victory.

During the winter transfer period of the 2020–21 season, Oostende received an offer for Theate of €5 million from Italian Serie A club Bologna, but the Belgian coastal club refused. In his first season at Oostende, he managed to qualify for European play-offs, but eventually missed out. He missed only one game due to suspension and four matches due to injury, playing the entirety of the other 27 matches.

In the 2021–22 season, Theate did not make an appearance during the first two matchdays pending a possible transfer. On the third matchday of the season, he returned to the starting line-up against Gent. On 22 August 2021, he picked up a red card in his last game for Oostende against his former club Standard. Theate played a total of 40 official matches for Oostende, in which he scored five goals.

On 29 July 2022, Theate joined Ligue 1 club Rennes on a four-year deal, for a reported fee of €20 million, making him the most expensive Belgian defender ever, above Thomas Vermaelen's 2014 move from Arsenal to Barcelona for €19 million. Theate was to succeed Nayef Aguerd who had left for West Ham United.

On the first matchday of the 2022–23 Ligue 1 season, Theate made his Rennes debut in the starting lineup against Lorient. He scored an own goal in the 65th minute, which proved to be decisive in the 1–0 loss. On 21 August, Theate scored his first goal for the club against Ajaccio, heading home the 2–1 winner. He also scored in his European debut with a strike from distance against AEK Larnaca in the UEFA Europa League group stage, as Rennes won the game 2–1. In Rennes' 1–0 league win over league leaders Paris Saint-Germain, Theate was praised for shutting down Lionel Messi in a strong performance.

On 14 September 2023, Theate signed a new contract with Rennes until June 2027.