As revealed by Goal, the Dutchman agreed to succeed David Moyes as permanent boss at Old Trafford last month and will assume control when his job as Netherlands coach ends after this summer's World Cup in Brazil.
But United has opted to wait until after the club's last Premier League fixture - away to Southampton on Sunday - to make the appointment official.
The Red Devils have endured a torrid season which saw Sir Alex Ferguson's successor David Moyes sacked after just 10 months in charge and Ryan Giggs handed the reins for the final four matches of the campaign.
Giggs remains unsure of his long-term role at the club beyond the summer, with the precise make-up of Van Gaal's backroom team at Old Trafford still to be revealed.
The 40-year-old brought himself onto the pitch for the final 20 minutes of United's 3-1 home win over Hull City on Tuesday evening, in which 18-year-old debutant James Wilson scored twice.
Last month Ferguson told a fundraising dinner in Manchester that Giggs is the right man to lead the club in the wake of Moyes' departure, but club owners the Glazers have gone with the more experienced choice of Van Gaal.
The decorated Dutchman has won league titles in three different countries and a Champions League title, and regards United as the perfect final chapter of a glittering managerial career.