Introduction Road traffic injuries (RTIs) are a major cause of both morbidity and mortality globally. Relative to countries with similar economic patterns both within and outside of South-East Asia, Cambodia's road traffic fatality rate is high, with motorcyclists accounting for more than half of all fatalities as a result of head injuries. Despite the initiation of national motorcycle helmet legislation for Cambodian drivers in 2009, helmet use among both drivers and passengers remains low. Methods This study adopted a two-pronged approach to assess the current status of and knowledge, attitudes, and practices (KAPs) towards helmet use among drivers and passengers in five provinces in Cambodia. The objective was to better understand helmet use over a two year period since the introduction of the 2009 legislation. Researchers conducted both (1) direct observation of daytime and nighttime helmet use (January 2011–January 2013) and (2) roadside KAP interviews with motorcyclists (November 2010–November 2012). Results The observed helmet rate across all study sites was 33% during nighttime and 48% during daytime, with proportions up to ten times higher among drivers compared with passengers. Self-reported helmet use was higher than observed use. Within the past 30 days, 60% of respondents reported that they “always” wore a helmet when they were drivers while only 24% reported they “always” wore a helmet as a passenger. Reported barriers for use among drivers included: “driving route”, “forgetfulness”, and “inconvenience/discomfort.” Conclusion Despite awareness of the protective value of helmets, motorcycle helmet use rates remain low in Cambodia. Many misconceptions remain in Cambodia regarding helmet use, including that they are unnecessary for short distance or at low speeds. These serve as an important barrier to helmet use, which, if dispelled and coupled with visible and regular enforcement, may significantly reduce the number of motorcycle-related injuries and fatalities.