Interfacial tension is a key parameter affecting industrially relevant properties of emulsions, such as morphology and stability. Although several methods are available to measure interfacial tension, they are based on generation of droplets starting from separate emulsion components and cannot directly probe the interfacial tension of an emulsion as such. Here, a novel microfluidic tensiometry device to measure interfacial tension of a water-in-oil emulsion in situ as a function of surfactant concentration is presented. In our approach, interfacial tension is obtained from a quantitative analysis of the deformation of individual emulsion droplets under steady state shear flow in microfluidic channels. The technique is validated by comparing the results with experimental data obtained by the pendant drop method in a broad range of interfacial tension values. A very good agreement is found, and an estimate of the surfactant critical micellar concentration (CMC) is also obtained. The proposed microfluidic setup can be used even at high surfactant concentrations, where the measurement is made more challenging by sample viscoelasticity, thus providing a powerful tool to determine the interfacial tension of complex systems in an extended concentration range. The technique could be also used for in-line monitoring of emulsion processing.