Cardiac arrest was Malaysia's highest cause of death for over two decades. All healthcare providers should be prepared to perform resuscitation, especially in an out-of-hospital setting. This research aimed to develop and validate a questionnaire to assess knowledge, attitudes, and self-efficacy on adult cardiopulmonary resuscitation (CPR), known as the KAECPR questionnaire among healthcare providers, focusing on Assistant Medical Officers (AMOs), nurses, and midwives in an out-of-hospital setting. Sixty-one items were initially developed according to the latest Basic Life Support (BLS) guidelines and experts' consensus involving four sections: demographic data and three domains on knowledge, attitudes, and self-efficacy towards adult CPR. This questionnaire was assessed regarding content validity, face validity, and internal consistency reliability. The average of content validity index at scale level (S-CVI/Ave) and universal agreement of content validity index at scale level (S-CVI/UA) showed over 0.80 for all domains. For average index of face validity (S-FVI/Ave) and universal agreement index of face validity at scale level (S-FVI/UA) also showed more than 0.80, indicating that the items scale was clear, relevant, and understandable. All three domains showed Cronbach's alpha values of over 0.70, indicating it was a reliable instrument. This study demonstrated that the newly developed questionnaire had achieved acceptable content validity, face validity, and internal consistency reliability. Therefore, this instrument can be used to evaluate the knowledge, attitudes, and self-efficacy of AMOs, nurses, and midwives in Malaysia regarding out-of-hospital adult CPR. The questionnaire is recommended to be used with existing life support courses in Malaysia to enhance learning and concept-grasping among healthcare providers.