Red calcite veins are found in Mesozoic limestones at several locations within the Transdanubian Range, western Hungary. Opinions on the origin and the formation conditions of the red calcites vary widely, with concepts ranging from high-temperature hydrothermal processes to speleothemic formation. The present study aims to constrain the formation temperatures and determine the fluid origins of the red calcites. Samples were taken from three locations previously investigated in earlier studies: the Sintérlap quarry at Sümeg, the Keselő Hill quarry at Tatabánya, and the Piliscsaba–Jászfalu quarry at Piliscsaba. Formation temperatures for the dark red calcites, presumably the earliest, ranged between 30 and 37 °C, as determined by the clumped isotopes method. The oxygen isotope compositions (δ¹⁸O) of the calcite-forming water were calculated using clumped isotope temperatures and the δ¹⁸O values of calcite. These calculations revealed a fluid that had either undergone high-temperature water-rock interaction or contained an 18O-enriched component and the temperatures are much lower than thought previously, but higher than those of speleothems. The hydrogen isotope compositions of inclusion-hosted waters, along with the carbon and oxygen isotope values of red calcites, suggest that the three red calcite occurrences were caused by distinct fluid movements, likely at different times. This indicates that a single red calcite formation event can be ruled out.