Critical thinking, a vital skill for the 21st century, has become a desirable educational outcome by educators. Promoting students' critical thinking has been a concern in various educational settings and the role of teachers is considered to be the most significant factor for successful instructional implementations. The present study aims to examine the effects of WebQuest-supported critical thinking instruction on the critical thinking disposition levels of Turkish pre-service teachers (PTs) of English and to determine whether receiving WebQuest-supported critical thinking instruction leads to a change in the PTs' understanding of critical thinking. Data were collected from 60 freshman PTs studying at a state university in Istanbul, Turkey. Thirty PTs were assigned to the experimental group and received WebQuest-supported critical thinking instruction, while 30 PTs were assigned to the control group and received traditional text-only instruction. The study lasted seven weeks and data were collected by means of the Turkish version of the California Critical Thinking Disposition Inventory and semi-structured focus group interviews conducted before and after the treatment. Findings revealed that the WebQuest-supported critical thinking instruction produced statistically significant differences in the PTs' critical thinking disposition levels and the PTs demonstrated clearer awareness of critical thinking at the end of the study.