Neymar scored for Paris St-Germain in their pre-season friendly against Jeonbuk Hyundai Motors
The deal for the former Barcelona player, 31, is understood to be worth about 90m euros (£77.6m) plus add-ons.
"I've achieved a lot in Europe and enjoyed special times, but I have always wanted to be a global player and test myself with new challenges and opportunities in new places," he said.
Neymar was not part of PSG coach Luis Enrique's plans for the new season.
The striker, who joined PSG for a world-record fee of £200m in 2017, was left out of their squad for Saturday's Ligue 1 draw against Lorient.
His exit also fits PSG's strategy of moving away from the 'Galacticos' era of signing high-profile players for big fees and significant wages, with Lionel Messi also leaving the club this summer.
"I want to write new sporting history, and the Saudi Pro League has tremendous energy and quality players at the moment," Neymar added.
"I have heard a lot and learned that I am following a long list of Brazilian players who have played in Saudi Arabia over the years, so I believe it is the desired place."
The forward was understood to be earning an annual income in the region of 25m euros (£21.6m) at the French club.
He will reportedly be paid 150m euros (£129.2m) a year in Saudi Arabia - six times the amount he earned at PSG - having signed a two-year contract.
Al Hilal's chairman, Fahad bin Saad, called Neymar "a global icon" and "an unstoppable attacking force" in welcoming him to the club.
The Brazilian made 173 appearances for PSG, helping the club win 13 trophies, including five Ligue 1 titles, as well as reaching the 2020 Champions League final.
"It is always difficult to say goodbye to an amazing player like Neymar, one of the best players in the world, said Nasser Al-Khelaifi, PSG's president and chief executive.
"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."
Neymar's departure comes two days after PSG's Kylian Mbappe was reintegrated into first-team training after positive talks with the club.
The France forward, 24, had been in a contract stand-off amid a desire to join Real Madrid but could now extend his contract at PSG, with his current deal due to expire next summer.
Mbappe was also left out of the squad for the Lorient game and watched the match from the stands.
Last month, PSG gave Al-Hilal permission to talk to Mbappe after the Saudi side made a world-record £259m bid.
Neymar's move continues a remarkable summer spending spree by Saudi clubs, underlining the league's ambition to be one of the top competitions in the world in the next few years.
Karim Benzema, N'Golo Kante, Jordan Henderson, Ruben Neves, Sadio Mane and Roberto Firmino have all moved to Saudi Arabia.
Portugal superstar Cristiano Ronaldo joined Al-Nassr from Manchester United in January.
Neymar's new team-mates include ex-Wolves captain Ruben Neves, former Chelsea centre-back Kalidou Koulibaly and Sergej Milinkovic-Savic, formerly of Lazio, who were all signed for big fees.
After Riyad Mahez left Manchester City to join Al-Ahli in July, Pep Guardiola said the Saudi league's financial muscle has "changed the market".
"A few months ago when Cristiano [Ronaldo] was the only one to go, no-one thought this many top, top players would play in the Saudi league," the City manager said.
"In the future there will be more and that's why clubs need to be aware of what is happening.
"I don't know how long they will sustain it, but the feeling is that they will stay."
-- Courtesy of BBC Sport