WASHINGTON—President Donald Trump’s former campaign chairman Paul Manafort registered Tuesday as an agent of a foreign government, acknowledging for the first time that a small part of his lobbying work for Ukraine’s former president was aimed at influencing U.S. policy makers and journalists.

The Justice Department filing by Mr. Manafort, who for years advised the ousted Ukrainian President Viktor Yanukovych, came after his work in Eastern Europe fell under scrutiny during the U.S. presidential election, leading to his resignation last year as Mr. Trump’s campaign chairman. His filing also acknowledged that his firm, DMP International, was paid more than $17 million for his lobbying work for a Ukrainian political party.

Ukraine was the longtime battleground where Russia and the West sought influence in the years before Moscow seized the country’s Crimean peninsula and Mr. Yanukovych fled to Russia in 2014.

Lobbying work on the behalf of foreign governments to influence political activities in the U.S. is tightly controlled by the Foreign Agents Registration Act. The law includes substantial criminal penalties for violations but hasn’t been aggressively enforced.

Mr. Manafort, a lobbyist and Republican operative who has cultivated an international client list over the years, is one of several aides to Mr. Trump facing scrutiny by the Federal Bureau of Investigation and several congressional committees, according to people familiar with the matter. Both Mr. Manafort and former Trump administration national security adviser Mike Flynn are being investigated for potential criminal violations of FARA, these people said.