ABSTRACT In Vietnam, the Ministry of Education and Training recently issued the 2018 Literacy Curriculum Guideline, which required elementary teachers to add different knowledge for teaching reading comprehension based on the competency-based approach. This study aimed to develop an instrument to assess pedagogical content knowledge for teaching reading comprehension based on this curriculum guideline. Participants were a group of four experts, 126 pre-service teachers in three universities of education, and 105 in-service teachers in ten elementary schools in Ho Chi Minh City. Using four stages of instrument development, the Pedagogical Content Knowledge for Teaching Reading Comprehension Instrument (PCK-RC) was developed, including 58 items. Data analyses found high levels of validity and reliability for the PCK-RC instrument. Moreover, data collection with larger samples and integration with interviews, and observations are recommended to measure teacher knowledge in reading comprehension more profoundly.