Although formative peer assessment (FPA) has become a prevailing learning activity in different educational settings, there are not enough suggestions on how to structure it with instructional supports in online environments to optimize its benefits. Therefore, this study aims to propose design guidelines for the development of an effective online FPA environment with instructional scaffolds in the context of writing activities at high schools. To this end, an online FPA environment was designed on the basis of an assessment model for regulated learning and teachers' and students' experiences. It was evaluated and refined three times. The formative evaluation findings suggested designing an online FPA environment with preparatory activities, information resources, goal setting and planning, anonymity, criteria form, sustainable and supportive dialog, motivational elements, and an easy-to-use interface. As a result, 11 design guidelines were produced. Overall, this research provides a better understanding of the essential design elements of online FPA environments.