Developing universities to become green and sustainable universities is important. This is in line with the world’s sustainable development guidelines. In developing a university towards being green and sustainable, different sustainability assessment criteria have been used, including the UI GreenMetric, GRI, STARS, AUN, THE’S Impact Ranking, and AISHE. Each criterion is designed in a broad-spectrum manner that is not specific. Therefore, this research aims to develop sustainable and green university assessment indicators that suit the national context in Thailand, both in terms of the size and location of universities. Based on the criteria from the UI GreenMetric, there are six categories, totaling 51 indicators. Thus, this study aims to analyze the weaknesses and strengths of each indicator using a questionnaire distributed by the Sustainable University Network of Thailand to all 36 universities and to organize a meeting with sustainability experts (focus group) in each area. To obtain appropriate indicators and to prioritize each indicator, the analytic hierarchy process (AHP) and weighted arithmetic mean (WAM) method were used to develop an index of sustainability assessment criteria (GU SI) suitable for the Thai context. The results of this study show that the newly developed GU SI comprises seven aspects, including a total of 27 indicators. These are criteria that can be used to assess the sustainability of small, medium, and large universities. They are not complex, are straightforward to use, and more importantly reflect the Thai context. By applying these new sustainability assessment criteria, it was found that the universities sampled in this study yielded significantly high scores. This study is limited by its application of sustainability assessments to only universities in Thailand. Though there are different contexts across different Thai universities, the proposed indicators can still be used to evaluate the sustainability of universities in Thailand.