Since its EU entry in 2013 Croatia has been recording increased rates of emigration, including to Germany.
Plenkovic attended a mass with Croat emigrants in Munich, where on Saturday and Sunday he participated in an international security conference.
"Half of EU countries have negative birth rates, and when the freedom of movement in Central and Eastern European countries is added to that, one feels the effects," the PM told reporters.
He added that the EU's cohesion policy had to enable countries like Croatia to catch up with the more developed member-countries.
"We will continue with the national demographic policy, housing policy, tax reduction policy and youth policy and I believe that all of that will help change the trends," he said.
Plenkovic promised that his government would continue fighting against corruption, which Croatians cite as one of the reasons for emigration.
"A lot has been done, but there are always certain cases that contribute to the general perception... that the fight against corruption must be stronger," he said.
"That will be our duty also in the next term," said Plenkovic, who this year will run both in intraparty and in parliamentary elections.