The Netherlands was the first country in the world to legalise gay marriage, and Amsterdam is known for its tolerance of same-sex couples.
King Willem-Alexander on Tuesday visited the Netherlands' main gay rights association to mark its 70th anniversary, in what was said to be the first such visit by a Dutch head of state.
The king met in Amsterdam with representatives from the organisation known as COC Netherlands, which has been advocating for gay rights since 1946.
It was an "immense honour for all LGBT people in the Netherlands and I think the world," said Tanja Ineke, the president of the association.
"When you're 13 and think you might be a lesbian and wondering whether to come out of the closet or not, and you see that the king is visiting the COC and supports you, then it's a huge boost," she told AFP.
The Netherlands was the first country in the world to legalise gay marriage, and Amsterdam, which hosts a huge annual gay pride event, is known for its tolerance of same-sex couples.
But Ineke said there was still "a lot to do in the Netherlands" and still "fights to fight," adding the COC was now advocating for a ban on discrimination against LGBT people to be included in the constitution.