University students are experiencing increasing rates of mental health difficulties, and digital mental health interventions (DMHIs) offer a cost-effective, accessible, and scalable way to support student wellbeing. However, engagement with these interventions is often lower in real-world settings than in research studies. Understanding the factors influencing engagement is therefore essential. This study explored the perspectives of university students with ongoing mental health difficulties regarding their use of and engagement with existing DMHIs. Interviews were conducted between August 2024 and February 2025, audio-recorded, and transcribed verbatim. Data were analysed inductively using reflexive thematic analysis and then deductively mapped to the Theoretical Domains Framework (TDF) to identify factors influencing engagement. Four TDF domains were identified as important determinants of engagement: knowledge, beliefs about consequences, goals, and environmental context and resources. Students highlighted limited awareness of available DMHIs, uncertainty regarding their effectiveness, and practical barriers to use. Findings provide important insights into the barriers and facilitators affecting student engagement with DMHIs and suggest that improving awareness, accessibility, and confidence in intervention effectiveness may enhance uptake and sustained engagement among university students experiencing mental health difficulties.