23 May 2016 Last updated at 18:35 BST
He's been given the boot after two years.
United finished fifth in the Premier League this season, which means missing out on the Champions League.
Winning the FA Cup on Saturday wasn't enough to save van Gaal.
Former Chelsea boss Jose Mourinho is now expected to take over at Old Trafford.

The official data prompted the government to upgrade growth forecasts for the second time in a week and to promise no new austerity measures.
Last week, growth for 2014 was up-rated to 3% from 2.1%, but will now be around 4.5%, Ireland's finance minister said.
Michael Noonan said the economy was in "a catch-up phase... after recession".
The year-on-year growth of 7.7% was the strongest recorded in Ireland since the early 2000s, Mr Noonan told reporters. By comparison, the eurozone recorded growth of just 0.7% over the same period.
Along with Greece, Spain and Portugal, Ireland was among the eurozone's worst hit economies, with the bursting of its property bubble and soaring unemployment.
The 1.5% quarterly rise beat the 0.5% expected by economists polled in a survey by the Reuters news agency.
Mr Noonan said the faster growth would mean Ireland's budget deficit falling to about 3.5% of GDP this year against a target of 4.8%. That meant 2bn euros of tax rises and spending cuts planned for next year would no longer be needed to reach the EU limit of 3% by the end of 2015, he said.
The economic improvement comes after years of austerity, a bailout from international lenders, and relentless austerity measures.
"At face value, the numbers suggest that the boom is back," said Austin Hughes, chief economist at KBC Bank Ireland.
"It shows a clearly improving economy. It's very encouraging to see the improvement in consumption, in construction. So a domestic recovery is taking hold, if a lot less intense that the headline number would suggest," he said.
Manchester United fans have been reacting to manager Louis van Gaal's sacking from the club.
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The Republic of Ireland's economy grew 1.5% in the second quarter of the year, figures show, and was up 7.7% on the April-to-June period in 2013.