Over the last decade, municipal solid waste generation in Phnom Penh has increased noticeably; however, the waste management system is far from satisfactory. Considerable amount of waste is left uncollected, as well as intentionally disposed of in public open spaces. External and internal factors can trigger these problems. Possible external factors are low collection frequency, low cleaning services, and insufficient facilities such as small dumpsters. Possible internal factors, which also play an important role in this issue, include low awareness, insufficient knowledge, and low responsibility for personal waste. To examine the influences of these internal and external factors on people’s waste disposal behaviors, we selected and conducted a questionnaire survey at four sites in Phnom Penh that differ in waste collection frequency and population density. A total of 413 valid responses were obtained. We developed a structural equation model to explain people’s intentions not to dispose of waste in public open spaces. The results showed that personal and social norms, such as perception of social pressure from friends and family and from the government, had significant influences on intention, whereas the influence of external factors was much smaller.