Arthur Nicolas R. Theate (born 25 May 2000) is a Belgian professional footballer who plays as a centre-back or left-back for Bundesliga club Eintracht Frankfurt and the Belgium national team.

Theate played youth football for Alliance Melen-Micheroux, Eupen, Genk and Standard Liège. Until reaching the under-14 level, he played as a striker at Eupen. At Genk, he was repositioned as a left winger after the arrival of a new player, after which he moved to the position of left-back and then centre-back. During his time at the Genk youth academy, his future at times looked uncertain, and he was described as being physically tough, but not always refined.

After three years playing for the Genk youth teams, he joined Standard Liège where he also struggled with regular midfielder Damjan Pavlović being played as left-back in the youth team to replace Theate. He soon returned to Genk, where he trained with the first team during the 2019–20 pre-season under head coach Felice Mazzu. After this, however, Theate failed to make the cut and was demoted to the reserves. He returned to Standard in 2019, but barely a year after his return, Standard let him go again.

In the summer of 2020, Theate saw no prospects of succeeding at Standard Liège, 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 26 August 2021, Theate signed with Serie A club Bologna on initial season-long loan, with a subsequent obligation to buy.