Purpose: Typical speakers tend to adopt words used by their conversational partners. This "lexical alignment" enhances communication by reducing ambiguity and promoting a shared understanding of the topic under discussion. Lexical alignment has been little studied to date in autism spectrum disorder (ASD); furthermore, it has been studied primarily via structured laboratory tasks that may overestimate performance. This study examined lexical alignment in ASD during discourse and explored associations with communicative success and executive function. Method: Thirty-one autistic and nonautistic adolescents were paired with a study-naïve research assistant (RA) to complete a social communication task that involved taking turns verbally instructing (guiding) the partner to navigate on a map. Lexical alignment was operationalized as the proportion of shared vocabulary produced by guides on successive maps. Task accuracy was operationalized as the pixels contained within the intended and drawn routes. Results: Results indicated that autistic adolescents had greater difficulty describing navigational routes to RAs, yielding paths that were less accurate. Alignment was reduced in autistic participants, and it was associated with path accuracy for nonautistic, but not autistic, adolescents. The association between lexical alignment and executive function missed significance (p = 0.05); if significant, the association would indicate that greater executive function difficulty was associated with reduced lexical alignment. Conclusions: These findings provide preliminary evidence of reduced lexical alignment in ASD in an unstructured discourse context. Moreover, positive associations between lexical alignment and task performance in the neurotypical group raise the possibility that interventions to promote the use of shared vocabulary might support better communication.