The aim of the present study was to determine species composition of ascidians and to detect the occurrence of alien ascidian species in Izmir Bay (Eastern Aegean Sea). The study was conducted at seven stations situated in recreational marinas and frequently used places for boating, fishing, and shipping activities in Izmir Bay between 25 and 27 August 2015 and between 23 and 26 November 2015. According to the findings, the non-native ascidian species such as Microcosmus squamiger Michaelsen, 1927, Didemnum ahu Monniot C. & Monniot F., 1987, and Polyclinum sp. were recorded for the first time on natural and artificial substrates at the sampling stations. The results indicate an increase in the number of alien ascidian species in the region. Also, the findings of this study showed that global marine invasive species M. squamiger has spread from the western Mediterranean to the eastern Mediterranean.