Insistence on Sameness has been pathologized as a subtype of restricted, repetitive patterns of behavior, interests, or activities, in autism. We examined the meanings of Insistence on Sameness for autistic adults through the lens of critical autism studies and environmental psychology, exploring their foodways and experiences of food environments, in relation to eating out and grocery shopping, and their experiences of autism (self-)diagnosis. Data were collected from earlier individual email interviews with Brazilian adults on the autism spectrum ( N = 16), either formally diagnosed in adulthood ( n = 10) or self-diagnosed ( n = 6). Our thematic analysis generated the following three themes: (1) Insistence on Sameness behaviors in food space as participants’ efforts for human–environment optimization—reducing uncertainty, uncontrollability, and exposure to unwanted sensory stimuli and social interaction; (2) suppressed Insistence on Sameness behaviors for being considered weird habits; and (3) realization of Insistence on Sameness as an authentically autistic way of coping when self-knowledge deepens through autism (self-)diagnosis. Our findings suggest autistic adults’ insistence on sameness is an authentically autistic way to appropriate space and exercise their right to comfortably co-exist and live as human beings and as themselves. Lay abstract Insistence on sameness is common in autistic individuals and continues into adulthood. Research shows it may be a way to cope with environments because of their sensory sensitivity, intolerance to uncertainty, and anxiety. Understanding the reasons for insistence on sameness from the perspective of autistic adults is important. To study the meanings of insistence on sameness for autistic adults, we interviewed 16 Brazilian autistic adults. All 10 formally diagnosed participants were diagnosed in adulthood. Six participants identified as being on the autism spectrum without formal diagnosis. During the interviews by email, we first asked about participants’ experiences with autism diagnosis, either formal diagnosis or self-diagnosis. Then, we asked about their experiences in places for eating out and grocery shopping. We found they tended to always go to the same places and use protective accessories to eat or shop comfortably. But their such behaviors were considered weird habits, first by other people and later by themselves. While trying to control their weird habits because of social pressure, they often suffered anxiety and meltdowns. When they finally learned of their autism in adulthood, they began to better understand who they are and why they experience the environment differently from others. This new understanding taught them that their so-called weird habits are actually part of their authentically autistic ways to cope with the weirder world. This study suggests that autistic adults’ insistence on sameness is an authentically autistic way to exercise their right to comfortably co-exist and live as human beings and as themselves.