A microwave method and a microwave device for measurement of water content in flowing crude oil are proposed. The method is based on measuring power of electromagnetic waves propagated through a transmission line and reflected from the load that is a TEM resonator, determining extrema of relative reflected wave power depending on frequency, and finding coordinates (relative power and frequency) of these extrema. Two modifications of the device are described for the measurement of water content within 0.01–30% and 0.01–100%. Their sensors are a coaxial line filled in with low-loss dielectric with sectorial flowed-through channel in it and a twisted two-wire line whose conductors are dielectrically-coated respectively. Results of laboratory experiments and tests in real conditions are presented.