WASHINGTON—President Donald Trump has signed off on a decision to provide Ukraine with lethal defensive weaponry, including portable antitank armaments, U.S. officials said Friday, in a major shift in the yearslong conflict between Kiev and separatists backed by Russia.

The decision to send Javelin antitank missiles places Washington closer to the conflict in eastern Ukraine. The administration notified Congress of the decision on Friday, U.S. officials said.

Ukraine...