Existing research on gender minority stress theory largely presumes that transgender identity is a categorically immutable characteristic often tied to a unidirectional gender transition, neglecting to consider individuals whose gender identity/expression and embodiment desires change over time. Applying constructivist grounded theory, this article empirically develops the concept of *detransphobia* from the distal and proximal gender minority stressors, stigma, and discrimination experienced by individuals who shifted or reversed their gender transition. Between October 2021 and January 2022, 28 participants completed semi-structured, one-on-one virtual interviews regarding their experiences of detransition/retransition and their social support needs. Interviews ranged between 50--90 minutes and they were transcribed and analyzed following an iterative, multi-pronged coding process to thematically conceptualize detransphobia. Fifty-two percent of the sample reported three or more past gender identities, 61% currently identified as nonbinary, and 100% identified along the LGBTQ+ spectrum. Participants' experiences of multiple gender transitions, and their embodiment following detransition, rendered them vulnerable to unique gender minority stressors for inhabiting an unintelligible category---detrans. Detransphobia was found to be rooted in cisnormativity and transnormativity, together with socio-politically-located anti-transgender stereotypes related to the process and the outcomes of detransitioning. Detransphobia compounds gender minority stressors and social exclusion in those who shift or reject their past transgender identity through the process of detransition.