The 55-year-old Dutchman replaces Xabi Alonso, who left Leverkusen to join Real Madrid.
Ten Hag has been out of job since getting sacked by United in October with the Old Trafford club languishing 14th in the Premier League table.
His most successful stint came at Ajax Amsterdam, who he coached from 2018-22, winning three Eredivisie titles and taking the club to the Champions League semi-finals in 2019.
Under Spaniard Alonso, Leverkusen secured the double in the 2023-24 season as they lifted their first Bundesliga title without losing a game, won the German Cup and reached the Europa League final.
Ten Hag had been linked with a return to Ajax before penning a deal with Leverkusen, who finished second to Bayern Munich in the Bundesliga this year. (Reuters)
