Wikis have been increasingly integrated into English-writing teaching and have served as one of the Web's most crucial social-networking tools, improving the writing of EFL learners worldwide. The purpose of this study has been to examine the positive effects of wiki-mediated collaborative writing and peer feedback on the grammatical and lexical accuracy of Saudi EFL learners' writing. To this end, 26 female secondary-school learners participated in the study and were selected from a private school in Riyadh. They were randomly assigned to a control group (N=12) and an experimental group (N=14). Both groups were given an identical writing task. However, in the control group, members individually completed the task, which was then reviewed by the teacher, while in the experimental group, members collaboratively completed the task, which was then reviewed by peers on a wiki platform. The data collection for this study rests on pre- and post-writing tests. The obtained data suggest that collaborative writing and peer feedback on wiki significantly improved the participants' grammatical, but not lexical, accuracy in their writing. However, the results of the post-test data also reveal that the experimental group outperformed the control group regarding the average total errors. The results confirm the efficiency with which wiki-based writing-oriented instruction can improve learners' accuracy.