The first cases of novel coronavirus disease (COVID-19) were reported in late December 2019 in Wuhan City, Hubei Province of China. It has spread rapidly around the world. Taking preventive measures by the public plays the most important role in infection prevention and control of the COVID-19. One of the major challenges in preventing and controlling COVID-19 has been the proliferation of misinformation on social media. In such a special epidemic, assessment of knowledge and attitude of the public will help to reduce the uncoordinated social behavior arising from misinformation. The purpose of this cross-sectional online survey was to determine the level of knowledge, attitude and practice (KAP) of Iranian population toward outbreak of COVID-19. Data were collected by self-administrated questionnaire. Analytical statistics were performed on the results. The mean scores of respondents’ KAP were 87.7, 59.3, and 89.6, respectively. Married and females participants, who work in healthcare systems, participants in the age group of 36 to 45 years with a higher level of education, and those who studied in the fields related to health gained significantly higher knowledge scores. Most respondents (63.7%) had a negative attitude towards the observance of appointed prevention principles by others. Also, the results showed a statistically significant positive correlation between all sections of KAP. Our findings suggest that despite the rapid dissemination of reliable and unreliable information about COVID-19 through cyberspace, the Iranian people have sufficient knowledge to deal with COVID-19 outbreak.