Research on the acquisition of American Sign Language (ASL) by deaf autistic children has documented similarities to the linguistic profile of hearing children on the autism spectrum and has identified sign-specific phenomena that could serve as clinical markers of autism in the deaf population. However, the acquisition of a signed language by deaf autistic individuals whose language use appears to be at age-level is not well documented. Here we present a case study of ASL use by a Deaf autistic adolescent exposed to ASL since birth. Data include two video recordings (collected at age 16;11 and 17;4 years) of his spontaneous ASL. Utterances were coded for repetitions, phonological form, fingerspelling, metalinguistic awareness, and facial expressions having grammatical functions. Other data include a writing sample (17;6) and background information obtained from parental interviews and formal psycho-educational evaluations (between ages 14 and 17). Results revealed this adolescent has strong ASL skills including sophisticated vocabulary and semantic content, advanced fingerspelling, and evidence of metalinguistic awareness. His distinctive signing patterns include abundant repetition of phrases, intrusion of non-ASL handshapes, articulation disfluency, reduced facial expressiveness, and a dissociation between fingerspelling and writing skills. The results of this study diversify and broaden discussions of the ways that autism may interact with language development, especially in the visual-gestural modality. Understanding language patterns in autistic signers more fully will improve identification of autism in the deaf population, promote acceptance of diverse signing patterns in the Deaf community, and lead to better support for this population.