There are a lot of differences between them.

Sirens originated from Greek mythology and are originally described as a combination of women and birds in various forms, usually as bird-like creatures with large women’s heads.

Mermaids, on the other hand, are described to have the head and upper body of a woman and a tail of a fish. They also appear in folklores in many cultures, from Europe, the Americas, Africa, and Asia.

Sirens are believed to cause sailors’ deaths. They lure them with their beautiful voices and kill them. A famous Greek folklore claims that Sirens would die if any mortal who heard them sing lived to tell the story.

Depending on cultures, Mermaids can also be depicted to lure sailors to their deaths with their voices and/or beauty but they’re mostly seen as gentle and peace-loving creatures who try to live their lives away from human interference but sometimes show affection to humans.

By the 14th century, the image of the Sirens became similar to that of the Mermaids, and they became interchangeable.