The size of droplets and bubbles, and the properties of emulsions and foams strongly depend on dynamic interfacial tension (γd) - a parameter that is often inaccessible due to the very short time scales for droplet and bubble formation, and the inaccessibility of (e.g., food) production lines. To solve this challenge, we developed a microfluidic tensiometer that can measure γd by monitoring the formation time of both droplets and bubbles. Our tensiometer is a pressure-driven microfluidic device that operates based on the principle of a pressure balance: the formation of a droplet (or a bubble) is initialized when the Laplace pressure of the interface is decreased below the externally applied pressure, and this decrease is caused by a reduction in γd that can be calculated from the applied pressure and the Young-Laplace equation. The decay of γd due to surfactant adsorption can be followed at the characteristic time scale, which is dependent on surfactant type and concentration. For 0.05-1% wt sodium dodecyl sulfate (SDS), we were able to measure γd at time scales down to 1 ms and 0.1 ms for droplet and bubble interfaces, respectively, at increasing applied pressures and SDS concentrations. Our tensiometer proves to be a simple, robust method that inherently allows access to nearly the full range of dynamic interfacial tension at relevant time scales.