Children with autism or attention-deficit/hyperactivity disorder (ADHD) are more likely than neurotypical children to experience eating difficulties, such as picky/selective eating, slow eating, and over/under-eating. Recent studies show that eating difficulties can persist into adulthood for autistic individuals. Yet little is known about the mechanisms involved and whether adults with ADHD also experience such challenges. We (1) compare eating difficulties among adults with autism, ADHD, autism/ADHD, and neither condition and (2) identify predictors of overall eating pathology and of food selectivity. The sample was 50% female and 78% White, consisting of 961 adults aged 19-57 who self-reported their diagnostic status: 184 with autism, 416 with ADHD, 292 with autism/ADHD, and 69 comparisons. Eating difficulties were assessed via self-report survey using the Swedish Eating Assessment for Autism (SWEAA) and Food Preferences Questionnaire. Autistic adults, with or without ADHD, reported significantly more eating difficulties than all other groups. Total eating difficulties were associated with increased insistence on sameness, other autistic features, food selectivity, and hyperactivity-impulsivity. Food selectivity was predicted by increased insistence on sameness, sensory sensitivity, and other core features of autism, and lower hyperactivity-impulsivity. The high degree of eating difficulties in autistic adults warrants clinical attention on this topic.