The performance of a hygrometer based on calibration-free direct tunable diode laser absorption spectroscopy (dTDLAS) for in-cylinder H2O measurements is demonstrated in a nearly unmodified production internal combustion engine. The H2O concentration is a proxy for the residual gas fraction remaining in the cylinder after intake-valve closure. One challenge for in-cylinder measurements, especially in multi-cylinder engines, is to obtain optical access to the combustion chamber. The measurements here were performed in the flywheel-side cylinder of a four-cylinder engine with small access ports that were previously designed for endoscopic imaging. Due to their position these ports prohibit the usual collinear arrangement of the optical elements typical for line-of-sight measurement techniques. Therefore, we developed a new “angled” fiber-optical interface, which allows a trans-illumination of the engine at a 90° angle. The optical fiber interface uses a scattering target inside the combustion chamber with its 84 mm bore achieving an absorption length of about 70 mm. With this arrangement, crank-angle resolved measurements of the H2O concentration during early compression could be realized with a temporal resolution of 250 μs and a H2O detection limit of 0.074 vol.%. This allows detailed analysis of single engine cycles as needed for residual gas investigations. Measurements were performed over a range of loads (25–100 Nm) and speeds (1400–3650 rpm), over which the residual gas fraction was expected to vary significantly. H2O concentrations were measured between 3.3 and 5.0 vol.%. The results were compared with a simple model of residual gas content and were found to agree within the combined uncertainty of both methods, which gives an indication that dTDLAS can be used to validate more complex engine models beyond what is possible by pressure-trace analysis.