The sea with the highest average salinity among the world's seas is the Red Sea.
The Red Sea averages about 40 practical salinity units (PSU), compared with roughly 35 PSU for typical seawater. Its northern waters can reach approximately 41 PSU, while the southern region is closer to 36 PSU.
This unusually high salinity results from intense evaporation, very little rainfall, and the absence of major rivers flowing into the sea. The Red Sea is also connected to the Gulf of Aden through the relatively narrow Bab-el-Mandeb strait, which limits the exchange and dilution of its water.
A common mix-up is the Dead Sea, whose salinity is far higher—around 300 PSU in some areas. Despite its name, the Dead Sea is a hypersaline lake, not a true sea. The Mediterranean is also salty, but its average salinity is lower, generally around 38 PSU.