Impostor phenomenon is defined as a psychological condition when some successful people do not fully ascribe their success to ability or competence, but attribute it to luck, generosity from others, or misjudgment, thereby experiencing an internal conflict. Microaggression is defined as subtle disparaging behavior that consciously or unconsciously discriminates people based on their background, personal identity, and group membership. Both impostor phenomenon and microaggression are commonly experienced in science, technology, engineering, and mathematics (STEM) fields, especially by women and BIPOC individuals—black, indigenous, or other person of color. Hence, the connection between microaggression and impostor phenomenon among BIPOC individuals needs deeper exploration. This qualitative study examined the research question: How do Hispanic/Latinx PhD students and postdoctorates in STEM describe impostor phenomenon and microaggression based on ethnic identity? U.S.-based participants were recruited using convenience sampling and snowball sampling. Semistructured interviews were conducted with 29 participants who self-reported experiencing impostor phenomenon. Interview transcripts were coded and analyzed inductively using constant comparison to develop themes. Twenty-two of the participants (18 women) experienced microaggression during training based on their Hispanic/Latinx identity. Microaggressive comments were made by faculty members, peers, and others in academia. Microaggression and impostor phenomenon were related through “othering” or feeling like outsiders, creating a sense of (un)belonging in STEM fields.