"No longer can the American taxpayer carry a disproportionate share of the defence of Western values," Mattis said in prepared remarks given to his counterparts at NATO headquarters in Brussels.
"If your nations do not want to see America moderate its commitment to this alliance, each of your capitals need to show support for our common defence."
Washington has long insisted that NATO members should spend two percent of their GDP on defence, a goal that few meet despite agreeing on it at a summit in Wales in 2014.
Consecutive US defence chiefs have hammered NATO on the topic for years, but Mattis`s call for cash -- and his blunt language -- carries extra weight as it comes after Trump while campaigning said US help for NATO allies might be contingent on how much they have paid.
"Americans cannot care more for your children`s future security than you do," he said.
Mattis called for "milestone dates" this year that would track NATO member contributions.
Currently, only the United States, Britain, Estonia, Greece and Poland have hit or surpassed the two percent figure.
"We must ensure we are not in the same spot at the end of the year that we are in today," Mattis said.