Vocabulary is a fundamental component of any language. Learning vocabulary plays a crucial role in learning English language. Whereas wiki is a promising Web 2.0 technology that can be used innovatively in EFL instruction. This study aimed at examining the effectiveness of wiki-based instruction on vocabulary learning of first secondary graders. It was carried out in Sabya, Jazan, Saudi Arabia in 2018. It followed the quasi-experimental design of one experimental group. Fifty-seven Saudi teenage EFL students participated in a researcher-designed wiki-based vocabulary course in a random male school. The course consisted of twelve lessons to teach 80 words picked from the grade's English Schoolbook (Mega Goal 2) to ensure their importance and benefit to students. Vocabulary pre-test and post-test along with a closed observation card were used to collect data. Descriptive statistics, paired samples and one-sample t-tests were used for analysis. It was found that wiki had slight positive effect on vocabulary learning. Results demonstrated that students achieved significantly better marks in their post-test but with a very low effect size (0.32). In addition, wiki was observed as usable, motivating, vocabulary enlarging assistant, and can be perceived positively by students. In the contrary, it was observed that the students' 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.