Abstract Background Despite a growing number of adolescents and adults diagnosed with autism spectrum disorder (ASD), little is known about service needs and barriers to services in this population. Existing research shows that youth with ASD are more underserved as they approach final years of their high school education and that adequate services for individuals with ASD after transition to adulthood are even scarcer. However, few studies have directly compared differences in service availability between adolescents and adults with ASD, and even fewer studies are published on service use outside Anglo-Saxon countries. The purpose of the present study was to examine service access, perceived barriers, and unmet needs, as reported by parents of adolescents and young adults with ASD in Poland. Methods The study used a subsample of parents of young people with ASD (aged 12–38 years; N = 311) from the Polish Autism Survey – a survey covering different areas of functioning of people with ASD in Poland, based on a convenience sample. Responding parents were recruited via different service providers, social media, and press, and completed a survey using a web platform or a paper-and-pencil questionnaire. Results As expected, adults used services less often than adolescents, with 80.1% of adolescents and 61.1% of adults with ASD using services in the previous 12 months. Mental health services were among the most used and the most needed services, followed by educational services, while needs for sensory/motor services remained largely unmet. Young people with a coexisting intellectual disability used more services than those without it. Non-governmental organizations, private clinics, and schools were the most common service providers. Parents indicated that most of young people with ASD had unmet service needs for services (93.5%) and faced barriers to access them (82.7%). Low-income families and those living outside large cities were at the highest risk of facing barriers to service access. Conclusions The results confirm still a thin body of evidence from different countries suggesting that adolescents and adults with ASD were both largely underserved populations. Policy-makers should address economic, regional, and age-related inequities in access to services for individuals with ASD.