Heat transfer measurements in transitional flat plate boundary layers subjected to surface roughness, strong pressure gradients, and freestream turbulence are presented. The surfaces considered consist of a smooth reference and 26 deterministic surface topographies that vary in roughness element aspect ratio, height, and density. They are designed to cover the full range of roughness regimes from smooth over transitionally rough to fully rough. For each surface, two pressure distributions, characteristic for a suction and a pressure side turbine vane, are investigated. Inlet Reynolds numbers range from 3.0 × 105 to 6.0 × 105 and inlet turbulence intensity is varied between 1% and 8%. Furthermore, different turbulence Reynolds numbers, i.e., turbulence length scales, are realized while the incident turbulence intensity is kept constant. Additionally, the turbulence intensity and Reynolds stress distributions in the freestream along the flat plate are measured using x-wire probes. Results show a strong influence of roughness and turbulence intensity on the onset of transition. The new data set is used to develop an improved correlation considering the roughness height, density, and shape as well as the turbulence intensity and turbulent length scales.