The White House says Biden called Arseniy Yatsenyuk (ahr-SEHN'-ee yaht-sehn-YOOK') Thursday.
It's an important signal of U.S. recognition for Yatsenyuk as the de facto leader of the former Soviet republic.
Biden told Yatsenyuk the U.S. will give full support as Ukraine seeks to restore order in the wake of a major political crisis.
The White House has said by fleeing the capital, President Viktor Yanukovych (yah-noo-KOH'-vich) abdicated power, but he reportedly still claims authority. Biden had spoken to him regularly during the crisis.
Yatsenyuk was overwhelmingly approved by Parliament Thursday. He's widely viewed as a technocratic reformer and has said Ukraine's future lies with the European Union.