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his study aimed at investigating the effectiveness of wiki-based instruction on vocabulary learning of third intermediate graders. It was carried out in Sabia, Jazan, Saudi Arabia in 1440/2018. It followed the experimental method using a quasi-experimental design of one experimental group. The sample consisted of 31 Saudi teenage students who studied selected words via a vocabulary wiki. The researcher designed a wiki-based vocabulary course of twelve lessons to teach 70 words picked from the third intermediate English Schoolbook (Super Goal 6) to ensure their importance and beneficialness to students. A fellow English teacher has taught the course. A vocabulary pre-test and post-test along with a closed observation card were used to collect data. Data were analyzed by descriptive statistics and one-sample t-test. The findings indicated that the wiki had slight positive effect on vocabulary learning. The results demonstrated that students achieved significantly better marks in their post-test but with a low effect size (0.48). In addition, wiki was observed as usable, motivating, vocabulary enlarging assistant, and can be perceived positively by students. In the contrary, the collaborative work level was low. Accordingly, wikis can be a good vocabulary teaching tool if well-designed and well-implemented after training both teachers and students.