Impostor phenomenon (IP) is an experience of psychological discomfort where some high-achieving people disbelieve their success. Those experiencing IP feel undeserving and fear being discovered as a fraud in one’s area of expertise. This study examined how early career researchers or ECRs of Hispanic/Latino origin in science, technology, engineering, and mathematics (STEM) fields described ethnicity-based experiences of IP. The research question examined how Hispanic/Latino ECRs (current PhD students and postdoctoral trainees) in STEM describe ethnicity-based experiences of IP during doctoral or postdoctoral training. Twenty-nine US-based ECRs were sequentially surveyed and interviewed. Participants were recruited purposefully and by snowball sampling through professional networks and social media. Descriptive statistics from surveys indicated that participants experienced moderate to intense IP at the time of the study with a mean score of 73.65/100 indicating high IP. Interviews with the same participants were coded and thematically displayed using constant comparison. The following themes were constructed: 1) family background and first-generation status, 2) disparity in observable traits and ethnic identity, 3) communicating in English, 4) enhance diversity, and, 5) underrepresentation and isolation. IP in connection with racial, ethnic, and other identities is poorly understood; culturally-informed understanding requires more research.