"I'm very unhappy with Spain. They're the only country that didn't raise their number up to 5%... so I'm not happy with Spain," Trump told reporters at the White House, Reuters reported.
"I was thinking of giving them trade punishment through tariffs because of what they did, and I think I may do that," Trump added.
Trump has repeatedly pushed NATO members to spend more on their own defense and cast doubt on Washington's willingness to come to the aid of members who do not spend enough. He said last week while meeting Finland's president that NATO should consider throwing Spain out of the alliance over its refusal to agree to the new commitment.
NATO members argue that their previous spending commitment of 2% of GDP is no longer sufficient.
Spain was the only member of the 32-nation alliance not to commit to increasing military spending to 5% of GDP.
Spanish Prime Minister Pedro Sánchez secured a last-minute exemption at the time, saying Spain would only spend up to 2.1%, which he called "sufficient and realistic."
Madrid, which joined the North Atlantic Treaty Organization in 1982, has argued it compensates for the lower spending with strong troop contributions to NATO missions, including deployments in Latvia, Slovakia, Romania, Bulgaria and Turkey.