Belgian foreign Minister Didier Reynders, a Francophone liberal, said the result sent a "powerful" signal to Europe.
Speaking at the US embassy in Brussels on Wednesday morning, he said he now expects greater protectionism from the US. Immigration and defence are the policy fields where the greatest changes are expected, said Reynders, adding that it was now time for Europe to do more for its own defence and in foreign affairs.
"The EU should strengthen its defence capabilities and make its voice heard in foreign policy."
Referring to a Trump presidency, he said "It will be difficult. We have seen his programme."
Belgian interior Minister Jan Jambon said the result throws up "new uncertainties," adding, "Think of Nato, climate agreements, free trade agreements. Question marks are now being placed.
"If you say we are going to review the US's Nato contribution, this is a process that will take some time, but we know that Belgium is not meeting its commitments under the present key. If our share rises under a new key, then we will certainly face problems."
International development Minister Alexander De Croo, also at the US embassy, asked who will provide experience in a Trump White House, saying, "With regard to the intention to build walls, limiting the scope of free trade accords and undoing climate agreements you will need people who can govern the country."
Attending the same post-election event, Luc De Vos, of the Belgian Royal Military Academy, told this website that he too believes that Europe must now be prepared to pay more for its own defence.
The academic said, "Europe can no longer go on thinking it can get away with paying only two per cent of national budgets on defence. That is going to change under a Trump presidency."
He also believed the Trump administration could cultivate more harmonious relations between the US and Russia, adding, "Putin and he seem to get on so this is now an opportunity to build bridges between the two countries.
"This result is part of a general movement we are seeing, which includes Brexit of course."
Further reaction came from BusinessEurope President Emma Marcegaglia who said, "After a divisive electoral campaign, we hope to see signals of reconciliation and reassurance rapidly.
"The US are and will remain the EU's most important partner in shaping a fair and rule-based globalisation to the benefit of all. Many examples in the past have demonstrated that free trade done in a fair way leads to peace and prosperity whereas isolation is the road to poverty and conflicts.
"The European business community will therefore continue to support a close strategic and economic partnership with the US and a deepening of transatlantic economic ties, including a fair TTIP.
"The 45th President of the US is an entrepreneur. We hope that his decisions will be driven by political and economic reason."
Elsewhere, Nato Secretary General Jens Stoltenberg congratulated Trump, saying, "We face a challenging new security environment, including hybrid warfare, cyberattacks, the threat of terrorism. US leadership is as important as ever.
"Our alliance has brought together America's closest friends in times of peace and of conflict for almost 70 years. A strong Nato is good for the United States, and good for Europe.
"Nato has responded with determination to the new security situation. But we have more work to do. And I look forward to meeting Trump soon, and welcoming him to Brussels for the Nato summit next year to discuss the way forward."