Although studies on peer feedback have identified several advantages for second language writing classes, the benefits of peer feedback to low-proficiency students have not been thoroughly investigated. This current study aims to determine how low-proficiency second-language students of English can improve their writing by giving and receiving peer feedback. Utilizing mixed methods, data collection and analyses were primarily conducted through posttest-only control-group design and focus group discussion (FGD). There were twenty university student participants randomly selected from two writing mixed-proficiency level classes. Framed under Vygotsky's sociocultural theory, an analysis of the gains in writing ability measured from formative and summative writing samples indicated that the low-proficiency students, specifically in the experimental group, made more significant gains in their writing over the course of the semester than the high-proficiency students in both groups. Even though the results generally showed no significant difference, except for higher error avoidance efficiency between experimental and control groups and among the high-proficiency students in both groups, a significant difference was observed between the low-proficiency students in both groups (control group M = 81.6, experimental group M = 89.8, control group SD = 8.3, experimental group SD = 9.5, t(4) = -3.23, p = 0.032). This leads to the conclusion that low-proficiency students benefit from scaffolded peer feedback.