Gualtherus Carel Jacob Vosmaer (Oud-Beijerland, August 19, 1854 - Leiden, September 23, 1916 ) was a Dutch zoologist.

In 1882 he became Anton Dohrn's assistant at his zoological station in Naples.

In 1889 he returned to the Netherlands and became assistant to Professor Ambrosius Hubrecht in Utrecht.

Later he became a private teacher and lecturer in Utrecht and in 1904 he became professor of zoology in Leiden.

[1] Vosmaer was a specialist in the field of sponges, describing many species.