The Bay of Belfalas was the remainder of the eastern edge of the Great Gulf that had divided Beleriand from the Lands to the South in the First Age. After the end of the War of Wrath a large section of western Middle-earth was drowned, and with the new coastline the Bay of Belfalas extended from Andrast to the Mouths of Anduin, and then south past Umbar to the unknown southern shores. The Bay of Belfalas was an important inlet for incoming ships. It served as a passageway for Corsairs coming from the Havens of Umbar in the South.
Most of the Rivers of Gondor flowed into the Bay of Belfalas, and it had many smaller bays and capes. The mouths of the Anduin and the Rivers Lefnui, Blackroot, Ciril, Ringló, Gilrain (which was joined by the River Serni), and Harnen all emptied into the Bay of Belfalas.
The Bay was named for the region of Belfalas , and in it lay the rocky island of Tolfalas at the mouths of the Anduin.
It probably corresponds to the Mediterranean Sea in the real world, because of the culture, commerce, and warm climate.