Answer:
As with many questions, it depends . . . the Russian Orthodox Church is headed by the Patriarch of Moscow and All Russia (the Russian Orthodox Church is often referred to as "the Moscow Patriarchate," or simply, "MP"). By "All Russia," of course, is meant all the lands that have been traditionally included in a Russian State. Ukraine has been included in Russian States for the past several hundred years. Ukraine only gained independence after the fall of the Soviet Union in the early 1990s.
The Russian Orthodox Church has several hundred, if not thousands, of parishes in the Ukraine, under the designation of "The Ukrainian Orthodox Church under the jurisdiction of the Moscow Patriarchate." This jurisdiction is rather loose-the Ukrainian Church under the MP is considered an "Autonomous Church"-all the Bishops, as well as the Metropolitan (head of this Church) are voted on in Ukraine by Ukrainian Bishops; the MP has the "right of refusal" if a candidate is considered unworthy, but in practice, this happens very rarely. So, the Ukrainian Orthodox Church/Moscow Patriarchate, is indeed in union with the Russian Orthodox Church.