Different fiber optic sensors have been used for groundwater temperature monitoring and the question is which one to choose for a particular study. In the field conditions it is sometimes difficult to determine how much error is introduced by the sensor placement technique, packaging or cross-sensitivity between temperature and strain. These factors were studied in a laboratory groundwater simulator during a heat tracing experiment. The performance of three fiber optic technologies was evaluated – distributed temperature sensing, fiber Bragg gratings and continuous fiber Bragg gratings. All sensors had comparable accuracy of around 0.2 °C and resolution smaller than 0.1 °C. Therefore, factors which need to be considered when choosing a sensor for groundwater temperature monitoring are spatial resolution, sampling frequency and possibility to measure absolute/relative temperature. The experiment also showed that strain effects can be introduced even when fibers have a loose tube packaging.