Despite the salience of transparency in policy and democracy debates a global measurement of transparency has always been missing. In its absence, measuring the impact of transparency on accountability and corruption for a large number of countries has been difficult, with scholars using more or less adequate proxies. This paper introduces a new measurement of real transparency—the T‐index—using 14 de facto components, based on direct observations of official websites in 129 countries and five de jure components, based on the transparency laws and conventions adopted. The resulting index is a measurement with very good internal and external validity and moderate precision. The paper argues that de facto transparency must be considered alongside de jure (legal) transparency if we are to judge the impact (or lack of) transparency against accountability and corruption, as a large implementation gap exists, in particular in poor countries, between legal commitments and real transparency. The T‐index has significant impact on both perception and objective indicators of corruption, including perceived change in corruption over time as measured by the Global Corruption Barometer. An analysis of outliers shows that high transparency alone is not sufficient to achieve control of corruption, especially in countries with low human development and poor rule of law, although transparency is a robust predictor of corruption with GDP controls. The data with all sources is available for download as T‐index 2022 dataset: DOI 10.5281/zenodo.7225627 and an interactive webpage developed for updates is available at www.corruptionrisk.org/transparency.