Despite increased risk for heat-related illnesses (HRI) among oil spill cleanup responders, little research has examined factors related to the issue. This study assessed occupational heat-related knowledge, perceptions, and barriers among responders during cleanup activities. A total of 65 responders completed an online survey which examined occupational heat stress during cleanup activities. Of the respondents, most had 25 or more years’ experience, worked for companies with 19 or fewer employees, were not classified as safety and health professionals, had a Bachelor’s degree or higher, and worked in the northern or central regions of the United States. While most respondents were knowledgeable of heat stress, the items in which respondents were least knowledgeable were: identifying the difference between heat exhaustion and heat stroke, the appropriate use of salt tablets, the effects of air conditioning on acclimatization, and previous heat-related illness (HRI) as a risk factor. For knowledge of heat stress, there was a significant difference in the employment classification scores for non-safety and health professionals and safety and health professionals. Respondents reported that they tended to perceive that heat stress can be severe and that HRI’s may affect workers. Regarding self-efficacy, most respondents felt confident in contacting emergency medical services for HRI, recognizing signs and symptoms of HRI, and knowing what to do if a coworker became ill. Oil spill cleanup responders are at high risk for HRI, injury, and death and findings illustrate the need to improve heat stress knowledge within training programs with emphasis on non-professionals.