Artificial intelligence (AI) has increasingly been used to support EFL writing throughout different stages of the writing process. However, less is known about how second-degree English language students perceive AI-assisted writing in fully online learning environments. This study explores second-degree English language students’ perceptions and experiences of using AI to develop their EFL writing skills in an online learning environment, focusing on perceived benefits, challenges, and concerns. The study adopted a qualitative research design and involved 10 second-degree English language students from Class 16F at Mekong University. Participants studied entirely online and had experience using AI tools for English writing tasks. Data were collected through semi-structured interviews and analyzed using thematic analysis following Braun and Clarke’s approach. The findings identified five main themes: AI as a rapid and accessible writing support tool; AI support for language development and writing revision; AI as a source of support in fully online learning; concerns about dependence, accuracy, and academic integrity; and students’ critical and responsible use of AI. Participants perceived AI as useful for idea generation, vocabulary development, grammar checking, revision, and proofreading, while recognizing risks related to overreliance, inaccurate suggestions, independent thinking, and academic integrity. The study suggests that AI can provide valuable support for EFL writing when students remain actively involved in evaluating and modifying AI-generated suggestions. The findings highlight the importance of guiding students to use AI as a supplementary writing resource rather than a substitute for their own thinking and writing.