The transfer sees West Ham United receive an initial £55m plus a further £5m in potential add-ons for the Netherlands international, whose departure had looked increasingly likely since the club's relegation from the Premier League in May.
Summerville, 24, spent two seasons at the London Stadium after joining from Leeds United in 2024, scoring eight goals in 56 appearances during his time with the Hammers. He was among the club's standout performers in the first half of last season, scoring in five successive outings during January and February, though it was not enough to prevent West Ham going down to the Championship.
The winger's stock rose further at this summer's World Cup, where he featured in four of the Netherlands' matches and scored in group-stage victories over Japan and Sweden, helping Ronald Koeman's side progress from their group before their eventual exit in the last 16.
His performances in the United States, Mexico and Canada attracted interest from several clubs, with Manchester United and Roma both credited with attempts to sign him before Al-Hilal moved decisively to complete the deal, seeing off competition to secure his signature on a long-term contract.
The sale brings in significant additional funds for West Ham following the club's relegation, coming shortly after midfielder Mateus Fernandes' £85m move to Tottenham Hotspur earlier in the summer.
Between the two departures, West Ham have recouped well in excess of £100m this window as they look to rebuild for an immediate return to the Premier League under their new management structure.
Summerville becomes one of the most expensive departures from an English club to the Saudi Pro League this summer, continuing a trend of leading Premier League and Championship talents moving to the Gulf state as Saudi clubs continue to invest heavily in established top-flight talent.