Clinical guidance documents play an important role in ensuring access to high-quality autism spectrum disorder diagnostic assessment practices. The objective was to perform a systematic review of professional association and government clinical guidance documents for autism spectrum disorder diagnostic assessment, analyzing their quality and content. The government search was limited to English-speaking, single-payer, publicly funded health systems. A quality appraisal was conducted by two appraisers using the Appraisal of Guidelines Research and Evaluation, second edition tool. A content analysis was conducted for recommended clinical personnel and psychometric tools. The 11 documents demonstrated higher quality in Scope and Purpose (mean: 90.1, standard deviation: 7.4) and Clarity of Presentation (mean: 82.8, standard deviation: 9.4) and lower quality in Applicability (mean: 43.3, standard deviation: 23.8) and Rigor of Development (mean: 52, standard deviation: 21.9). All documents either recommended multidisciplinary team assessment or stated it was ideal. The documents varied substantially in their recommended tools and personnel for diagnostic assessment. There was little supporting evidence for team and personnel recommendations. Multiple guidance documents exist for autism spectrum disorder diagnostic assessments, with varying quality and recommendations. The substantial variation likely stems from insufficient evidence supporting assessment practices. Research is required to close the evidence gaps and inform high-quality clinical guidelines.