Critical Thinking has been identified in the Common Core State Standards (CCSS) and Next Generation Science Standards (NGSS) as skills needed to prepare students for advanced education and the future workforce. In science education, argument-based inquiry (ABI) has been proposed as one way to improve critical thinking. The purpose of the current study was to examine the possible effects of the Science Writing Heuristic (SWH) approach, an immersion argument-based inquiry approach to learning science, on students' critical thinking skills. Guided by a question-claims-evidence structure, students who participated in SWH approach were required to negotiate meaning and construct arguments using writing as a tool throughout the scientific investigation process. Students in the control groups learned science in traditional classroom settings. Data from five data sets that included 4417 students were analyzed cross-sectionally and longitudinally. Yearly critical thinking gain scores, as measured by Form X of Cornell Critical Thinking Test, were compared for students who experienced the SWH approach versus students who experienced traditional instruction in both elementary (5th grade) and secondary schools (6th-8th grades). Analyses of yearly gain scores for data sets that represented a single year of implementation yielded statistically significant differences favoring SWH over traditional instruction in all instances and statistically significant interactions between gender and grade level in most instances. The interactions revealed that females had higher gain scores than males at lower grade levels but the reverse was true at higher grade levels. Analyses from data sets that included two years of implementation revealed higher overall gains for SWH instruction than for traditional instruction but most of those gains were achieved during the first year of implementation. Implications of these results for teaching critical thinking skills in science classrooms are discussed in detail.