Background: Suicide is a leading cause of death among youth ages 15 to 29. This study identifies suicidal ideation (SI) subtypes among university students based daily reports of SI, assesses how stress sensitivity may affect SI variability within these subtypes, and how they differ in terms of past and future self-injurious thoughts and behaviors (SITB). Methods: 756 students participated in a 14-day ecological momentary assessment and web-based survey at baseline and 12-month follow-up. Latent Profile Analysis identified SI subtypes using indicators of intensity, variability and frequency. Multinomial regressions evaluated the associations between SITB and SI subtypes, as well between SI subtypes and future SITB. Linear models assessed how stress sensitivity was associated to SI variability within the subtypes. Results: Three SI subtypes were identified: sporadic and low intensity/variability (S1), frequent and medium intensity/variability (S2), and frequent and high intensity/variability (S3). Stress sensitivity was highest in S2 and S3, in S3 higher stress sensitivity was associated with lower day-to-day variability. Nearly all aORs for SITB significantly increased from S1 to S3 compared to controls, with each subtype showing higher aORs compared to the previous subtype (S3 vs, S2 vs S1). S3 students showed higher persistence of suicidal thoughts and behaviors at follow-up (aOR = 20.6; 95 % CI = 8.3-51.2), with 4 of the 7 attempts occurring among students in this subtype. Conclusions: Daily SI severity predicts future active SI and attempts, with stress sensitivity contributing to more extreme, rigid suicidal thinking. Targeting stress sensitivity could be effective for suicide prevention among university students.