Along Romania’s Black Sea shore, several coastal lakes form a unique natural landscape where freshwater and saltwater meet. These lakes, separated from the sea by narrow sandbars or dunes, are not only striking geographical features but also important ecological systems. Among the best known is Lake Techirghiol, famous for its highly saline water and therapeutic mud, used for centuries in health treatments. Its mineral-rich environment supports specialized plant and animal life, while also drawing visitors seeking natural remedies. Just north of Constanța lies Lake Siutghiol, a large lagoon separated from the sea by a thin strip of land on which the resort of Mamaia has developed. Siutghiol’s brackish waters are home to fish species that sustain local fisheries, and its shores are important resting places for migratory birds. Nearby, Lake Tăbăcărie, although smaller and surrounded by urban areas, plays an important role in the city’s environment. Despite pressures from human activity, it remains a green oasis for birdlife and a recreational space for locals. Further north, Lake Tașaul reflects the delicate balance of these ecosystems. Once connected to the sea, it has become a semi-closed basin where salinity and water levels change with the seasons. It shelters a variety of aquatic species and provides habitats for birds, but it is also affected by agriculture and pollution, making its conservation a pressing issue. Together, these coastal lakes form a natural corridor that supports biodiversity, protects the coastline, and sustains local communities. Their constant changes—sometimes saltier, sometimes fresher—create environments of great biological richness. At the same time, they are fragile, vulnerable to urban expansion, tourism, and climate change. The Romanian coastal lakes of Techirghiol, Siutghiol, Tăbăcărie, and Tașaul remind us that the boundary between land and sea is alive and ever-shifting, and that protecting these waters means preserving both nature and heritage.