In optical fiber networks it is important to monitor water which seeps into splice enclosures. The fibers have residual stress at splicing points and, when water is present, this adversely affects fiber lifetime.We developed a water sensor with optical fiber which has a simple structure for monitoring water at splicing points. This water sensor causes optical loss due to fiber bending when water seeps into splicing enclosures. The design method using a fiber bending model and sensor performance are described.