This article presents an empirical evaluation of the validity and reliability of a peer-assessment activity to improve academic writing competences. Specifically, we explored a large group of psychology undergraduate students with different initial writing skills. Participants (n = 365) produced two different essays, which were evaluated by their course peers and professors using a specific grading instrument (rubric). The validity of the task was demonstrated by a high inter-grader agreement and a strong degree of consistency between the ratings of the peers and professors. Although all students did not improve their writing skills between their first and second essays, the peer-assessment activity enhanced the writing abilities of low but not high-achieving participants. The pedagogical implications of these results are discussed.