Despite mounting evidence that women in foreign policy often bear the brunt of online hostility, the extent of online gender bias against diplomats remains unexplored. This paper offers the first global analysis of the treatment of women diplomats on social media. Introducing a multidimensional and multilingual methodology for studying online gender bias, it focuses on three critical elements: gendered language, negativity in tweets directed at diplomats, and the visibility of women diplomats. Our unique dataset encompasses ambassadors from 164 countries, their tweets, and the direct responses to these tweets in 65 different languages. Using automated content and sentiment analysis, our findings reveal a crucial gender bias. The language in responses to diplomatic tweets is only mildly gendered and largely pertains to international affairs and, generally, women ambassadors do not receive more negative reactions to their tweets than men, yet the pronounced discrepancy in online visibility stands out as a significant form of gender bias. Women receive a staggering 66.4% fewer retweets than men. By unraveling the invisibility that obscures women diplomats on social media, we hope to spark further research on online bias in international politics.