One of the most unique characteristics of the Baltic Sea is its low salt concentration.

Whereas the salt concentration of the planet's oceans is approximately 3,5 %, the salt concentration of the Baltic Sea is below 1 %. There are many reasons for this difference. 

One of the reasons is the large catchment area of the Baltic Sea, which results in salt-free rainwater flowing into the sea from a vast area. 

However, the most important cause for the low salt concentration of the Baltic Sea is that it is connected to the planet's oceans only via a narrow waterway. This causes only a small amount of salt-rich ocean water to enter the Baltic Sea. 

The salt concentration of the Baltic Sea is highest in the strait of Skagerrak, located between Denmark and Norway. This part of the sea is approximately 700 meters deep, making it the deepest area in the Baltic Sea. When moving towards the center of the Baltic Sea from Skagerrak to the strait of Kattegat, the seafloor rises drastically. In many parts of Kattegat, the water is only 50 meters deep. This causes most of the salt-rich ocean water to remain on the bottom of the Skagerrak strait. 

The salt concentration continues to grow lower when moving from Kattegat towards the inner parts of the Baltic Sea. The remaining salt-rich ocean water must pass through the shallow and narrow Danish straits to reach the main pool of the Baltic. After passing through these straits, the salt concentration of the water is only approximately 1 %. After the island of Bornholm, the salt concentration of the Baltic Sea becomes only 0,8 %.

When moving towards the north, the salt concentration continues to decrease, reaching 0,1–0,3 % in the Gulf of Bothnia and the Gulf of Finland. 

The water type of the Baltic Sea is called brackish water. It is a mixture of fresh and salt-rich water. The Baltic Sea receives large amounts of fresh water through the numerous rivers located in its vast catchment area. 

Because of the low salt concentration in the gulfs of Finland and Bothnia, these parts of the Baltic Sea freeze over more easily than regular seawater. The winter ice poses problems for both humans and the living organisms of the Baltic Sea. For humans, the most important challenge posed by the frozen-over sea is the difficulty it causes for shipping and boat traffic. 

The brackish water of the Baltic Sea accomodates fewer species than the planet's oceans do, and the total number of living organisms inhabiting the Baltic Sea is likewise relatively low. However, the organism community of the Baltic Sea is quite unique, consisting of both saltwater and freshwater species.

Different species have adapted to different parts of the Baltic Sea. For example, the starfish and the crab do not survive in the brackish water found in the pool of the Baltic Sea, and they are only found in the straits of Denmark and the southernmost parts of the Baltic Sea, where the salt concentrations are higher. 

Some freshwater species also inhabit the Baltic Sea. For example, the vendace can be found in most parts of the Gulf of Bothnia. Overall, freshwater species prefer the low-salt brackish water of the Gulf of Bothnia and Gulf of Finland, whereas saltwater species thrive in the southernmost parts of the Baltic.