The equivalent reduction in greenhouse gas emissions (GHGEs) by Mahayana Buddhists with vegetarian diets is quantitatively evaluated. The Buddhists in seven Mahayana-dominated countries or regions, i.e., China, Japan, Vietnam, South Korea, Taiwan, Hong Kong, and Singapore, are studied. Assessments of the vegetarian population among these Mahayana-dominated countries or regions are performed. Correlation formulas based on data from a national survey are developed to quantify the GHGEs of various dietary groups by using the meat consumption as the only required input. To demonstrate its reliability, the prediction from the formulas developed is first compared with the results of a food production-and-consumption study using a different approach. Then, the formulas are used to assess the GHGE reduction due to Mahayana Buddhists with vegetarian diets. The assessment indicates that Mahayana Buddhists with vegetarian diets account for the equivalent GHGE reduction of 48.83 million metric tons of carbon dioxide equivalent, which is a huge amount and is equal to 11.3 or 8.9% of the GHGEs from France or the UK in 2012, respectively.