The beaming monarch spent the day with his family in the southern city of Tilburg where he was feted by 50 pianists and a colourful dance performance.
About 150,000 orange-clad revellers joined the celebrations in Tilburg, the Dutch news agency ANP said, quoting the local municipality.
Some had stood in place in the streets since dawn hoping to shake hands with the monarch, it said.
The king, wearing a dark suit teamed with an orange tie, was shown on his way to Tilburg on a train accompanied by Argentine-born Queen Maxima and their three daughters in his new Instagram account, launched on his birthday.
Some 70,000 people celebrated the event in the streets of Amsterdam, ANP reported.
The king appeared in a television interview on the eve of his birthday which was watched by more than 4.2 million viewers.
Teary-eyed, he spoke of the pain of losing brother Prince Friso in a skiing accident in 2013 and the anguish it caused their mother, Queen Beatrix.
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