Research reveals that although teachers in English as second language (ESL) classrooms are often good at teaching language forms and skills, many are less confident about the cognitive engagement required to develop critical thinking skills. However, little research has been done to examine the development of critical thinking in the context of second-language learning and teaching. In this article, I report the results of a study that assesses the effectiveness of a program called Philosophy in Schools (PIS) in developing students' critical thinking and English proficiency in Hong Kong. In the study, I provided training and support for five teachers in two schools so that they could teach PIS to their secondary 3 and 4 ESL students during integrated humanities and English lessons respectively. I found that PIS played an important role in promoting the students' critical thinking and enhanced the development of their English proficiency to a significant extent.