Generative artificial intelligence, such as Chat Generative Pre-Trained Transformer (ChatGPT), has been widely used in second-language (L2) settings. However, its role in secondary L2 student writers’ academic transition is still under-researched. Meanwhile, the responses generated by prompting ChatGPT for personalized learning still need improvement. More specifically, ChatGPT-based assistance primarily concerns feedback on student writers’ language forms (i.e., grammar and word choices). This assistance still lags behind the needs of advanced L2 writing when secondary L2 students transition to academic contexts (e.g., higher education). Effective writing involves the relationship between contextually appropriate language resources and the content of a text. To bolster secondary L2 student writers’ academic transition to an advanced level in this digital era, this chapter demonstrates why and how ChatGPT-assisted L2 writing development can be optimized through students’ exposure to instruction and their leveraging of Halliday’s (1993) systemic functional linguistics (SFL), a theory that explicitly conceptualizes and unpacks meanings (e.g., the content of writing) with close attention to the role of language forms. This chapter begins by introducing the pedagogical affordances of ChatGPT for secondary L2 students’ writing. Subsequently, it moves to the literature review, arguing that, for the students’ academic transition, the affordance could be further improved by furnishing their knowledge of the relationship between language forms and content construction. Proceeding from the SFL perspective, the literature review discusses its role in supporting L2 students’ academic performance and explores its potential for combined use with ChatGPT in aiding secondary L2 student writers’ academic transition. The chapter then profiles strategies and examples of the integrated affordances of SFL and ChatGPT in preparing secondary L2 students for their academic transition to advanced writing. Overall, the chapter underscores the importance of a synergistic role for ChatGPT and SFL in conceptualizing and constructing meaningful texts, supporting secondary L2 writers’ transition to advanced writing in the digital era.