Cthylla (nickname: the Secret Daughter of Cthulhu) is a fictional character, a Great Old One from the pantheon of H. P. Lovecraft's Cthulhu Mythos. Originally from Xoth, she currently resides in Yhe, where she is guarded by Cthulhu's minions, so she will be able to give birth to Great Cthulhu again when he is destroyed in the distant future.

Cthylla's physical appearance is that of a giant, many-eyed octopus with small wings; like her father, she is able to alter her body proportions at will, though, so her wings can be extended until allowing her to fly. She is essential for Cthulhu's plans, and is thus constantly guarded by countless Yuggs, Yuggya and Deep Ones.