Invasive marine species, introduced largely through human activities like global shipping, aquaculture, and the aquarium trade, can profoundly disrupt ocean ecosystems. They displace native species, alter food webs, and trigger cascading ecological and economic impacts. Notable examples include the comb jelly Mnemiopsis leidyi in the Black Sea, which decimated fish stocks, and lionfish in the Atlantic and Mediterranean, which reduce reef fish diversity. Meanwhile, non-native seaweeds such as Caulerpa taxifolia smother native seagrasses, and the European green crab ravages shellfish populations. The resulting losses harm fisheries, tourism, and local economies. Prevention—via stricter ballast water treatment, regulated aquaculture, and early detection—remains the most effective and cost-efficient strategy to safeguard marine biodiversity.