The proliferation of sex/gender/sexuality, or phet (เพศ), categories in Thailand from the mid-twentieth century onwards is made legible through the production of rigid taxonomies of the so-called Thai sex/gender system. Rather than further solidifying Thai phet categories, this dissertation sets them in motion by bringing unprecedented focus to the messy and intensely social histories of thom (pronounced like the English tom: ทอม) and thrans man (pronounced like the English trans: ทรานส์). Popularized in the 1970s, the Thai term thom is used for and by women and others assigned or assumed female who embrace masculinity in their everyday lives. Far from a discrete category, thom incorporates and intersects with multiple gender positions, including phu-ying (woman; ผู้หญิง), phu-chai (man; ผู้ชาย), and thrans man, the latter of which has circulated in Thai vernaculars since the 2010s. Though distinct and diverse in their respective modes of embodiment and self-description, the subject positions of thom and thrans man often travel together in vernacular discourses and oral sources, tracing vibrant social lives that come into view, this dissertation argues, when we consider how gender and sexual diversities stand and sway in relation, rather than opposition, to alternative, additional, or seemingly contradictory gender and sexual formations, both normative and nonnormative. These relationalities take shape within thom and thrans men’s situated recollections of and their engagements with kinship structures, community, religious practice, work, education, and everyday discourses, which this dissertation describes as the wider social worlds of thom and thrans man in Thailand.