The additional use of selective catalytic reduction has increased the amount of SO3 generated by coal-fired power plants. Accurate measurement of SO3 is of great significance for SO3 removal and boiler operation. This work focuses on development and evaluation of an automated continuous monitor designed for SO3 measurement in coal-fired flue gases. This instrument is based on selective absorption of SO3 in isopropanol and the spectrophotometry determination method with data acquired in real time. Considering that flue gas from a typical power plant could contain high levels of particulate matter (PM), moisture, and SO2, which could interfere with the accuracy of SO3 measurement, targeted modifications were made in the design of sampling, determination, and liquid circulation equipment. The accuracy and transient response of the monitor were evaluated in a pilot-scale experimental system with SO3 concentrations from 0 to 60 ppm. The interference of SO2 in the accuracy of this monitor was also evaluated, and it was shown that the error caused by SO2 absorption and oxidization was <0.15% of the SO2 concentration. Comparison experiments with the manually controlled condensation method were conducted at high PM concentrations, low PM concentrations, and high SO2 concentrations, and they showed that results obtained by the two methods were almost the same. A full-site field application conducted with real flue gas in boilers that burn both high-sulfur and low-sulfur coals showed that the monitor was feasible.