Relevance & Research Question: The central aim of our research is to provide a reliable and valid scale to measure the degree of voluntary disclosure of private information (socially exhibitionistic behavior) on the internet and especially social media platforms. The relationship between self-disclosure on the internet and several measures of individual differences such as the Big Five, sensation seeking, depression, and social support is examined. We further examine the independence of online self-disclosure from offline self-disclosure. Correlations with objective measures of internet usage are also assessed. Methods & Data: Three studies were conducted with heterogeneous samples of internet users recruited online and offline. Study 1 served to develop a reliable 15-item scale and an 8-item short version. This was followed by an extensive questionnaire validation study to investigate both discriminant and convergent validity (study 2) and a quasi-experimental study comprising extreme prototypes of socially exhibitionistic behavior (study 3). Results: The findings strongly suggest that the new scale is an appropriate and reliable instrument for the measurement of socially exhibitionistic behavior in online environments. Moderate correlations with related constructs (e.g. extraversion, disinhibition, self-monitoring) were found while the independence of online self-disclosure from its offline variant was confirmed. Somewhat counter-intuitively, high self-disclosure on the internet did not correlate with depressive symptoms. Men scored higher than women on the newly developed scale. Added Value: With the new scale, self-disclosure on the internet can be measured more reliably than with ad-hoc questionnaires. We provide first insights into the relationship of online self-disclosure and established measures of personality. We also make a number of suggestions for future avenues of research. For instance, we encourage further investigations concerning the causal direction of these relationships. The connection of self-disclosure with the hot topic of privacy on the internet also promises fruitful new opportunities for research. The newly developed scale of socially exhibitionistic behavior can contribute to this.