"Donald Trump, as I've said before, is a dealmaker and I think that could be a good thing for Britain but it can also be a good thing for Europe," Johnson said as he arrived for an EU foreign ministers meeting, having snubbed special talks on the issue the night before.
"I think we all need to wait and see what they come up with, but I think we should regard it as a moment of opportunity," he added.
Trump's shock election win has sparked deep fears in Europe after his campaign-trail rhetoric appeared to cast doubt on US security commitments to NATO.
EU foreign policy chief Federica Mogherini called a special dinner late Sunday to discuss the EU's response but Johnson refused to attend, saying it would send a wrong signal to the new president.
Mogherini said after Sunday's talks she looked forward to a "very strong partnership" with the Trump administration, but warned that the EU could not just simply wait to see what he came up with.
"For the moment it's not a wait-and-see attitude we can afford having because the world goes on, Europe goes on, crisis goes on, but also opportunities we can take go on," she said.
Mogherini is pushing a new Global Strategy to give the EU a real defence policy, but Britain and several other EU member states oppose her plans because they believe they will undercut US-led NATO which has guaranteed Europe's security since 1949.
Johnson told reporters that increased EU defence cooperation and spending was positive, but it should not come at NATO's expense.
"It's important when you're setting up these EU structures that they should be complementary with NATO," he said.
"You shouldn't undermine the fundamental security architecture that's looked after us for the last 70 years."