The number of the studies conducted on the use of wikis on the English as a foreign language (EFL) learning process has remained fairly limited. More specifically, in the Turkish EFL context, little attention has been paid to the effects of wikis on EFL writing achievement. Thus, this study aims to examine the effects of a wiki-based writing environment in terms of EFL writing achievement in the Turkish EFL context. In this experimental study, a background questionnaire, a writing achievement pre- and a post-test were administered to a sample group of 42 EFL learners. Results indicate that the use of wiki-based online writing environment increases writing achievement regarding content. It is recommended that wikis as appropriate tools to increase learners' EFL writing achievement should be integrated into the EFL learning process. The advantages that wikis serve should not be ignored but exploited by the teachers.