Several measures have been developed to assess loneliness in the general population. Here, we examined if, and how accurately, widely-used measures capture the experience of loneliness in autistic adults. A total of 203 autistic participants from the UK completed an online survey, which included two widely-used loneliness questionnaires: the University of California, Los Angeles (UCLA) Loneliness Scale Version 3 and the Social and Emotional Loneliness Scale for Adults (SELSA). Participants completed the measures and provided their views on how well they captured their experiences of loneliness. A direct measure, asking participants how often they felt lonely, was also completed. Results demonstrated that scores on the UCLA and SELSA questionnaires positively correlated with the direct measure of loneliness, suggesting that these tools accurately index autistic experiences of loneliness. However, qualitative responses suggested that participants found both scales difficult to complete. Using reflexive thematic analysis, we identified three themes that encapsulated participants' views: (1) failing to distinguish between the characteristics/experiences of loneliness and being autistic, (2) loneliness is not a static trait and (3) inappropriate wording. Given that our sample of autistic participants tended to prefer the UCLA scale to the SELSA, we provide initial recommendations around how the UCLA scale could be adapted for autistic adults.