In-cylinder flow velocity measurements using particle image velocimetry (PIV) have been performed for the first time in a full-size marine Diesel engine. The engine was a four cylinder two-stroke engine with a bore diameter of 0.5 meter and a stroke of 2.2 meter. For such engines uniflow scavenging is used, with fresh air entering through angled ports at the bottom of the cylinder to generate a swirling flow and burnt gases exiting through a centrally located exhaust valve at the top. For efficient design of this process and for validation of CFD models it is essential to obtain an experimental characterization of the flow inside a fully operational engine. Optical access was obtained through a custom designed engine cover, fitted with a number of optical ports into which sapphire windows were mounted. Both the laser and camera used for PIV were mounted directly onto the engine in order to minimize effects of vibrations on optical alignment. Laser sheet generation and relay imaging modules were then positioned inside the optical ports, in order to achieve efficient illumination and detection. Zirconium oxide particles, introduced through the scavenging box, were employed for seeding. Velocity measurements could thus be performed over the entire engine cycle, except during peak combustion. Tangential and axial velocity components were measured at two different radial locations and for two different scavenging port configurations. The results obtained provide the first quantitative experimental information on swirl velocities in this type of engine.