With former President Donald J. Trump and his allies signaling that he will scale back assistance to Ukraine if he retakes the White House, a onetime close adviser has accepted a $50,000-a-month contract to lobby for Ukrainian interests.

Kellyanne Conway, who served as a top aide to Mr. Trump’s 2016 campaign and his presidency, registered on Friday to represent Victor Pinchuk, a Ukrainian oligarch who has positioned himself as a leading supporter of his country’s defense against Russian aggression.

In a filing with the Justice Department under the Foreign Agents Registration Act, Ms. Conway indicated that the Ukrainian government would be the “principal beneficiary” of her lobbying on behalf of Mr. Pinchuk’s charitable foundation.

The filing signaled that Ms. Conway would engage U.S. lawmakers and opinion leaders to explain the importance of Ukraine to the protection of democratic principles, and would try to raise awareness “of Ukrainians’ fight for freedom and the Russian illegal war of aggression.”

Ms. Conway also indicated that she would try to persuade American officials to attend the forthcoming installment of Mr. Pinchuk’s annual security conference, scheduled for next month in Kyiv.

Hours after the filing was uploaded to the Justice Department’s FARA website, Ms. Conway posted a photograph of herself on social media with Mr. Trump in New Jersey, saying that she had “quite the visit today” with him.