The England midfielder is the latest big-name player to move from the Premier League to Saudi Arabia after 12 years at Anfield.
The 33-year-old has finalised a deal with reported wages of up to seven hundred thousand pounds-a-week.
Liverpool is due to receive an initial twelve million pounds from the Saudi club.
Henderson led the Reds to their first English league title for 30 years in 2020, a year after lifting the Champions League.
The England international will be reunited with former Liverpool team-mate Steven Gerrard, who is Al-Ettifaq manager.
