Instructors use a variety of online formative assessment (FA) activities to support learning outside class. Previous studies have revealed barriers for students in online courses, but little is known about the barriers students experience when completing online FA assignments. Understanding these barriers to access is critical to fostering more inclusive learning for all students. Using a framework from previous work in online learning, we examined student perceptions of online FA access with respect to five barrier categories: technical resources, instructor organization, social interactions, personal engagement, and learning environment. We developed and administered a survey to more than 1200 undergraduate biology students at 2-year and 4-year institutions. Students responded to statements using Likert scales and open-ended prompts. Statistical models indicated differences in access across the barrier categories and revealed that demographic characteristics were associated with certain barrier categories. Furthermore, technical resources, instructor organization, and personal engagement barriers were associated with lower course performance. In open-ended responses, students most frequently suggested that changes to scheduling logistics, course delivery, and FA format would improve their online FA experience. We discuss how these findings and student suggestions can inform instruction, particularly how instructors can alter their FA characteristics to better suit their student populations.