The invention provides a humidity sensor and a manufacturing method thereof. The humidity sensor comprises a substrate, an electrode structure, and a humidity sensing structure. The electrode structure is disposed on the substrate. The humidity sensing structure is disposed on the electrode structure. The humidity sensing structure includes a first humidity sensing layer and a second humidity sensing layer. The first humidity sensing layer is in direct contact with the electrode structure and has a first oxygen vacancy number. The second humidity sensing layer is disposed on the first humidity sensing layer and has a second oxygen vacancy number. The second oxygen vacancy number is greater than the first oxygen vacancy number.