Tools to measure autism knowledge are needed to assess levels of understanding within particular groups of people and to evaluate whether awareness-raising campaigns or interventions lead to improvements in understanding. Several such measures are in circulation, but, to our knowledge, there are no psychometrically-validated questionnaires that assess contemporary autism knowledge suitable to the UK context. We aimed to produce a brief measure to assess between-respondent variability and within-respondent change over time. A pool of questionnaire items was developed and refined through a multi-stage iterative process involving autism experts and a lay sample. Attention was paid to face validity, clarity, consensus on correct responses, and appropriate difficulty levels. Initial validation data was obtained from a lay sample of 201 people. Difficulty and discrimination ability were assessed using item response theory and low-performing items were removed. Dimensionality was evaluated with exploratory factor analysis, which revealed a one-factor structure of the questionnaire. Further items were removed where they did not load strongly on their main factor. This process resulted in a final 14-item questionnaire called the Knowledge of Autism Questionnaire-UK. Internal consistency was satisfactory, and the final questionnaire was able to distinguish between parents of autistic people and those without an affiliation to autism. The KAQ-UK is a new, freely-available measure of autism knowledge that could be used to assess between-respondent variability and within-respondent change over time. Further evaluation and validation of its measurement properties are required.