Former Chelsea manager, Jose Mourinho who was sacked by Chelsea in December, 2015 will be taking over form his former boss, Van Gaal.
United board are unsatisfied with the poor Premier League campaign which saw them finished fifth, subsequently missing out on Champions League qualification.
The coaching staff of the Dutchman; assistant coach Albert Stuivenberg and goalkeeper coach Frans Hoek - have also been sacked.
The former Barcelona coach was pictured this morning arriving United’s training ground with employment lawyer Paul Gilroy QC, who has long worked with the League Managers Association.
Despite several reports of his sacking, a defiant van Gaal was already looking forward to next season at a news conference after the FA Cup final win over Crystal Palace.
"I show you the Cup," he said. "And I don't discuss it with my friends of the media, who already sacked me for six months. Which manager can do what I have done? I don't want to talk about leaving the club."
Van Gaal preferred to dwell on the positives from a season in which United won their first trophy since 2013 and brought in some promising young players but came one place lower in the Premier League, finishing fifth behind Manchester City on goal difference and missing out on the Champions League.
There has also been regular criticism from the club's supporters about a style of play that brought only 49 goals in the Premier League -- their lowest total since 1989-90.
"I said when I started that we were in transition," Van Gaal added. "I had to refresh my squad and I've done it. In spite of the many injuries this season we were still in the (Premier League) race and now we have lifted the Cup. That's a title.
"I'm very proud that I'm the first manager after the period of Sir Alex Ferguson (to do it)."
"Every year we want to improve. It is not so easy when United come for a player. When United come the price is double or much more."