They must pay their bills, said Trump in an interview in the influential Sunday program Meet the Press on the American TV company NBC.
He referred to NATO member countries that had not fulfilled agreements on economic support to the alliance.
Republican Trump had already warned during his first presidential term that American support to NATO would be reduced if other countries' budget promises were not fulfilled.
In the interview on Sunday, Trump said that member countries must "treat us fairly".
He also commented on the domestic political issue of undocumented migrants' status in the USA. Trump said that he "must" deport all those who do not have correct residence permits in the country.
