In his message to world leaders, he addressed those who slated the decision to leave the European Union.
"Some think Britain’s best days are behind us. To those people I say: you are gravely mistaken," the prime minister said.
Leaders of seven of the world's richest countries are gathering to talk about the global challenges and economic policies they face, including climate change.
The prime minister said the summit in Biarritz, France, comes at a critical moment for the world including where violence is trapping countries into poverty, girls' rights to education and international trade barriers.
The group of seven consists of Canada, France, Germany, Italy, Japan, the United Kingdom, and the United States.
Mr Johnson said: "My message to G7 leaders this week is this: the Britain I lead will be an international, outward-looking, self-confident nation."
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The PM continued: "We will be an energetic partner on the world stage. We will stand alongside our G7 allies to solve the most pressing international issues."
He added that the "family of nation-states" must reaffirm their universal values underpinned by democracy.
"Britain will never flinch from the special responsibilities that go with being a pillar and an architect of the international system," he said.
The prime minister continued: "Some people question the democratic decision this country has made, fearing that we will retreat from the world.
"Some think Britain’s best days are behind us. To those people I say: you are gravely mistaken.
"We will stand up for liberty, democracy, the rule of law, equality and human rights – the ideals that we share with our friends and allies.
"We will remain at the heart of the alliances that span the world."