A detailed overview of the challenges linked to the introduction of non-native species into the Caspian Sea is provided. The discussion covers these key areas: principal characteristics of the Caspian Sea’s physical geography, hydrology, and hydrochemistry; the origins and makeup of its native flora and fauna; the composition of non-native biota and its ecological consequences; the population dynamics of Mnemiopsis leidyi and its impact on the ecosystem; the role of Beroe ovata as a biological control agent for M. leidyi; and anthropogenic pressures affecting the Caspian Sea’s plant and animal life. Throughout the twentieth century, environmental conditions in the Caspian Sea declined significantly, primarily because of sea level fluctuations related to river runoff management and increasing pollution. The reconnection of the Black and Azov seas with the Caspian Sea following the completion of the Volga-Don Canal in 1952 allowed zoobenthic fouling organisms and macrophytes to spread into the Caspian. By the early 1980s, with the adoption of ship ballast tanks, phytoplankton and zooplankton also began arriving from the Black Sea. The establishment of most non-native species led to profound transformations within native biological communities. The Caspian Sea example illustrates how the rapid proliferation of gelatinous carnivorous invaders can swiftly disrupt the entire ecosystem’s functioning.