As chairman of the Council of the EU, he visited the border together with Greek PM Kyriakos Mitsotakis, European Commission President Ursula von der Leyen, European Council President Charles Michel and European Parliament President Davide Sassoli.
They arrived to support Greece given its efforts to protect its own and thereby the external EU border, preventing illegal migration.
"We have sent the message that the EU and all member states will support and stand in solidarity with Greece," said Plenkovic.
The European leaders flew over the border area in a helicopter to see the situation on the ground. They were joined by Croatian Interior Minister Davor Bozinovic, who will chair an extraordinary meeting of EU interior ministers on this topic on Wednesday.
The Greek army, police and PM Mitsotakis briefed them on what is being done to protect the border.
"It's important that Greece, with its security forces, prevents a possible new illegal migration wave," said Plenkovic. He and his colleagues also visited the Greek police barricades on foot.
He said Greece would receive support from Frontex and a rapid border intervention team as well as funding for that.
He said the situation was similar to 2015 and 2016 but with a key difference. "This Greek government, unlike the previous one (led by Alexis Tsipras), is controlling its border."
"After today's briefing, I believe Prime Minister Mitsotakis and the Greek authorities will guard their border," Plenkovic said.
EU leaders met in Kastanies amid increasing EU concerns about the arrival of thousands of migrants to the Greek border, after Turkey said it "opened its door" for refugees because dozens of Turkish soldiers had been killed in northern Syria.
Turkish President Recep Tayyip Erdogan said on Monday he had refused €1 billion in European aid for holding migrants in Turkey because the EU did not wish to "share the burden".
Text: Hina