The unelected European Commission president made the plea after claiming the EU’s current rate of expansion is “mediocre”.
Speaking as he collected an honorary degree in Salamanca, Spain, Mr Juncker said: “Europe, which we thought was very big, is actually very small.
“When Europe has all the possibilities, let’s take advantage of them, especially to finish everything that we have already started.
“And let’s give much more strength and vigour to this growth that I still qualify as mediocre.
His comments are likely to anger Eurosceptics who believe the bloc’s push for deep integration has already gone too far.
The EU has faced a wave of populist backlash from disgruntled voters across the bloc since last year’s stunning Brexit vote.
But he insisted: “I want to tell the traditional parties that populism will end up defeated. It can not advance.”
The Commission president went on to attack Catalonia’s bid to break away from Spain.
He said: “Nationalisms are a poison that prevent Europe from working together.
“We cannot stay with our arms crossed because it is time for us to do what needs to be done.
“I say ‘no’ to any form of separatism that weakens Europe and further widens the existing fissures.”
The secessionist drive has dragged Spain into its worst political crisis since the return of democracy four decades ago.
Spanish Prime Minister Mariano Rajoy, who was with Mr Juncker in Salamanca, thanked him for his support “in such difficult times”.
He said: “Juncker has supported the Spanish government’s actions, has expressed his respect for our constitution, our laws and our legal system.”
Additional reporting by Maria Ortega