Joseph Kabila, who took over the Democratic Republic of Congo at age 29 after the assassination of his father in 2001, announced on Wednesday he would not stand for re-election, raising hopes the country might carry out a peaceful transition of power for the first time in its modern history.

Mr. Kabila had long overstayed his welcome as leader, and for two years, Congolese opposition groups, the African Union, the United States and the United Nations had all urged Mr. Kabila not to defy term limits and seek a third term in voting scheduled for December.

This week, The Financial Times reported that the Trump administration was prepared to tighten financial sanctions against Mr. Kabila and his associates if he did not agree to relinquish power. In June 2017, the Treasury Department imposed sanctions against Gen. François Olenga, one of Mr. Kabila’s most senior military officials, and in December it put forward sanctions against Dan Gertler, an Israeli businessman who is close to the president.

Opposition activists rejoiced at the president’s decision, but also cautioned that it was only a first step toward an orderly transition of power.