The Covid-19 pandemic revolutionized the world's status quo to transition from face-to-face into online E-learning. However, this unexpected transformation in teaching-learning approaches necessitates teachers to teach while frequently assessing students' performance through online assessment, which has been substantiated to expose teachers to an array of unanticipated challenges. To this end, the current study was a bid to unearth the Iranian EFL teachers' perceptions of online assessment and illustrate the challenges encountered through adopting a sequential mixed-methods design. For the quantitative phase of the study, one hundred EFL teachers submitted responses to an online questionnaire constructed in Google Forms. The researcher recruited twelve teachers for the qualitative phase of the study from a pool of one hundred participants who completed questionnaires to participate in a semi-structured interview protocol. The findings from the descriptive statistics of the questionnaire revealed that Iranian EFL teachers had either a negative or a neutral attitude toward online assessment. In addition, the results of the interview offered more insight into the challenges that teachers encounter during online assessment. In this light, the three most frequent challenges were the high risk of students cheating and plagiarism, the issue of internet connectivity, and poor technological infrastructures. The findings of this study bear witness to the voices of a group of Iranian EFL teachers about their perceptions and challenges of online assessment during the COVID-19 epidemic. In the spirit of the findings, the recommendations and suggestions for further investigations are discussed.