Knowledge of indoor ventilation airflow is essential for creating a healthy, comfortable and energy-efficient indoor climate in buildings, airplanes, cars, ships, etc. In the past decades, numerical methods such as Computational Fluid Dynamics (CFD) have become primary tools to assess indoor airflow. However, validation of numerical simulations by high-quality experimental data is imperative. Although a lot of studies have focused on experimental data for turbulent ventilation flow, there is a lack of experimental data for transitional ventilation flow. Transitional ventilation flow is in general associated with relatively low air velocities inside the enclosure. This paper presents detailed Particle Image Velocimetry (PIV) measurements and an analysis of transitional flow in a ventilated enclosure. The isothermal forced mixing ventilation flow is driven by a transitional plane jet. The measurements are performed in a reduced-scale water-filled model for slot Reynolds numbers ranging from 800 to 2500. Flow visualizations indicate that this range of Reynolds numbers results in transitional flow, including the development of large coherent structures in the outer region of the jet. The measurements are analyzed in terms of mean velocity near the inlet and in the entire flow domain, and in terms of turbulence intensity and vorticity. Specific attention is given to the Coanda effect by which the free plane jet develops into a wall jet. The experimental data and analysis are specifically intended to support the development and validation of numerical models for ventilation flow.