Despite the importance of teachers’ assessment expertise for successful language teaching and learning, previous research studies reported insufficient or inefficient training in assessment (e.g., Vogt & Tsagari, 2014; Lam, 2015). While there have been efforts to provide better training opportunities, language teachers and teacher educators were faced with another challenge: the forced transition to online teaching (i.e., emergency remote teaching) in response to the COVID-19 pandemic. This study aimed to explore teachers’ assessment practices and learners’ perceptions in that specific online context. To offer valuable implications for teacher education, the graduate teaching assistant (GTA) group was highlighted in the investigation. The three overarching research questions guided this study: 1) What were language teachers’ practices and perceptions regarding online assessment? 2) What were the challenges and opportunities faced by GTAs regarding online assessment? 3) What were language learners’ perceptions and challenges related to online assessment? The data were collected quantitively (online surveys) and qualitatively (semi-structured individual interviews). The survey participants included 124 teachers and 97 learners in language courses offered online in Fall 2020 at US universities. The interviews were conducted with eight GTAs, three language program coordinators (who were responsible for teacher training), and eight learners. The findings suggested that teachers made modifications in different aspects of assessment (e.g., medium, weight), explored new methods (e.g., open-book exams, audio/video-recording), and utilized different feedback types (e.g., typed feedback, audio-recorded feedback) in the online assessment. Teachers expressed the need for more hands-on training opportunities specific to language teaching/assessment. GTAs were faced with challenges in time management for fulfilling their responsibilities but had the opportunities to engage in critical reflection on assessment. Compared to face-to-face (F2F) assessment, teachers’ concerns about academic dishonesty (cheating) were higher and more learners agreed that cheating would be easy in online assessment. To ensure academic honesty, teachers reported the use of various measures (e.g., plagiarism detector, video proctoring). Learners appreciated the additional individualized feedback they received in the online environment but wanted more interactive and helpful assessment activities that involved authentic language use. Implications for teacher education and recommendations for language programs, program coordinators, and teachers are provided.