The integration of generative artificial intelligence (AI) tools such as ChatGPT has transformed writing instruction by providing immediate assistance in idea generation, organization, and language refinement. While these capabilities may enhance writing performance, concerns remain regarding their potential influence on students’ critical thinking skills, particularly among learners who rely heavily on AI-generated outputs. This study examined the influence of ChatGPT on the critical thinking skills in writing of Grade 10 students in the Schools Division of Calbayog City, Philippines. Specifically, it categorized students as reliant or non-reliant users of ChatGPT, described their demographic profiles in terms of number of available gadgets, internet connectivity, socio-economic status, and geographic location, determined the relationship between ChatGPT reliance and these demographic variables, and compared the critical thinking skills in writing of the two groups. A descriptive-comparative research design was employed involving 342 Grade 10 students selected through stratified random sampling from sixteen public secondary schools. Data were collected using a researcher-developed ChatGPT Reliance Scale and the Paul and Elder Critical Thinking in Writing Rubric. Descriptive statistics, Chi-square test of independence, and Mann–Whitney U test were used for data analysis. Results revealed that most participants were classified as ChatGPT-reliant learners. Among the demographic variables, only socio-economic status demonstrated a statistically significant relationship with ChatGPT reliance, whereas the number of available gadgets, internet connectivity, and geographic location showed no significant association. Furthermore, non-reliant learners demonstrated significantly higher levels of critical thinking in writing than reliant learners. The findings suggest that although ChatGPT can serve as a valuable instructional support tool, excessive reliance may hinder the development of independent reasoning and critical thinking in writing. The study underscores the need for structured and ethical AI integration in English instruction and supports the development of policies and instructional practices that promote responsible AI use while strengthening students’ critical thinking skills.