<p>X (formerly Twitter) provides a social media platform for teacher collaboration. This study comprehensively maps the educational Twittersphere—sets of education-related communities—in Germany (<i>N</i> = 2,761,579 tweets, <i>N</i> = 143,004 users) between 2007 and 2022, serving as a blueprint for researchers seeking to study informal teacher learning at scale in other national contexts. We introduce a novel, reproducible method to discover, label, and examine national-level teacher communities using large-scale Twitter data. In doing so, we address a significant gap in the literature, as most prior research on online teacher learning has focused on English-speaking contexts, especially the United States. We describe both quantitative and qualitative interaction patterns across subcommunities, including participation and engagement trends of teachers and non-teachers over time. Focusing on Germany’s largest education-related community—Twitter’s Teacher Lounge (#twlz)—we examine how it reflects the characteristics of a Community of Practice (CoP), a model of informal learning with well-established links to effective teacher development. Our findings suggest that teacher participation in online CoPs is widespread, sustained, and characterized by shared domain language, community interaction, and collaborative practice. This evidence contributes novel understanding to why online teacher professional development is effective for teacher learning.</p>