The 31-year-old has joined from Paris St Germain in a deal that could go up to 100 million euros (£86.3m).
The Brazil forward add to the list of stars of world football to turn his back on a major European club, following the likes of Cristiano Ronaldo, Karim Benzema, Riyad Mahrez, Ruben Neves, Sadio Mane and N’Golo Kante to the Middle East.
The move ends a six-year stay at PSG, who he joined from Barcelona for a world-record £200m fee in 2017.
He leaves the Parc Des Princes having scored 118 goals in 173 matches and won five Ligue 1 titles.
Neymar was left out of Luis Enrique’s squad for the opening league game of the season and did not feature in the Spaniard’s plans.
PSG chief executive Nasser Al-Khelaifi told the club’s official website: "It is always difficult to say goodbye to an amazing player like Neymar, one of the best players in the world
"I will never forget the day he arrived at Paris St Germain, and what he has contributed to our club and our project over the last six years. We had a great moment and Neymar will always be a big part of our history.
"I would like to thank Neymar and his family. We wish Neymar all the very best for the future and his next adventure."
Neymar joins former Wolves captain Neves and ex-Chelsea defender Kalidou Koulibaly at Al Hilal.
Additional reporting from PA
