The purpose of this study was to investigate the extent to which wiki writing improves college students' grammatical knowledge in Spanish as a second language (L2). Fifty-three students participated in the study for three consecutive semesters. Pre- and posttest results show that wiki writing was helpful in improving their grammatical knowledge. In addition, students found designed wiki writing useful and helpful for developing their writing skills and L2 learning; however, they felt uncomfortable editing other students' writing and doubted the accuracy of others' editing. Discussion and implications for using wiki writing are provided.