In biogas reactors, it is vital to monitor the condensation of water vapor with increasing humidity. The use of fiber optic-based sensors that allow this control to be done online quickly and reliably facilitates the process. In this study, a fiber-optic water vapor and condensation sensor was designed and implemented for biogas reactors. It enables the detection of water due to water vapor and condensation that will increase humidity level based on the combination of fiber reflection losses and longitudinal alignment losses. By creating a very low-cost sensor mechanism that can react quickly and is not affected by environmental variables other than the parameter to be detected, the increasing water vapor and the initial moment of the condensation can be detected with high precision and speed.