On 15 August 2018, Russian club CSKA Moscow announced that Vlašić joined them on loan for the 2018–19 season. Three days later, he made his league debut in a 3–0 victory over Arsenal Tula. In his Champions League debut on 19 September, he scored both goals for CSKA Moscow in a 2–2 draw with Viktoria Plzeň. On 2 October Vlašić scored the only goal in a 1–0 home victory against the ruling champions Real Madrid in the Champions League. In the return leg on 12 December, he provided Arnór Sigurðsson with an assist for the third goal in a 3–0 victory, which is Madrid's highest ever European home defeat. Despite this, the Muscovites ended last in their group crashing out of the tournament. On 28 April 2019, he was sent off in a 2–0 defeat to Krasnodar. Vlašić finished the season with eight goals and seven assists.