The rise of AI-assisted writing tools has transformed academic literacy, as it offers students support in grammar correction, sentence structure, and idea generation. However, concerns about academic integrity and over-reliance on AI are on the increase. This study investigates how Nigerian university students use AI writing tools and examines their effects on writing proficiency and ethical considerations. A quantitative survey design was employed, with 350 undergraduate students from five public universities in northern Nigeria participating. Purposive sampling was used to select students familiar with AI-assisted writing. Data were collected via an online questionnaire and analyzed using descriptive statistics and a chi-square test. Results show that most students (75%) used AI-assisted writing tools, mainly for grammar correction (85%) and sentence structuring (70%). While 65% believe AI enhances their writing skills, fewer students reported improvements in structural organization. Ethical concerns were found significant, with 47% of students associating AI use with plagiarism and 49% focus on its impact on originality. The findings suggest that while AI tools support technical writing aspects, their excessive use may weaken students' independent literacy development. The study recommends structured AI integration in writing instruction to balance technology use with critical thinking. Future research should explore specific discipline differences in AI-assisted writing and its long-term effects on students' writing proficiency.