Objectives: The present study sought to qualitatively evaluate third-year undergraduate dental students' perceptions of sources of stress during the transition from preclinical to clinical training., Methods: This was an observational, cross-sectional, and qualitative study with a nonprobabilistic sampling design. The sample consisted of students in the third year of the five-year undergraduate degree program in dentistry offered by the São Paulo State University (UNESP), School of Dentistry, Araraquara, São Paulo State, Brazil (n = 37). Data were collected using open-ended, semistructured, and individual interviews that were recorded on a digital voice recorder. The students were interviewed on campus at a previously scheduled time. Efforts were made to provide a secure and welcoming environment for the interview. The interview questions addressed students' adaptation to clinical training and their perceptions of stress resulting from this transition. Data analysis was based on the qualitative and quantitative Discourse of the Collective Subject (DCS) technique performed with the aid of Qualiquantisoft®., Results: Most of the students evaluated (75.7%) reported difficulty in the transition from the preclinical to the clinical phase of their program and that this difficulty increased their stress levels during this transition (81.1%). The most frequently cited reason for the increase in stress was the responsibility and demands associated with caring for real patients (54.1%). Almost half of the students (48.6%) reported feeling physical symptoms of stress such as tachycardia, dizziness, headache, and muscle tension during this period. Most of the students (81.1%) required up to one semester to feel that they had adapted to the clinic. Many students used stress reduction strategies such as simply trying to calm down, studying before their clinical work, improving organisation, and asking professors for help. Adaptation to ergonomics and biosafety in the clinic was the most frequently cited impediment (45.9%) to the adaptation process. The students' main suggestions for reducing stress in this transition phase were additional preclinical training in a clinical setting, a more gradual transition, and greater professor receptiveness., Conclusion: The dental students interviewed herein perceived high levels of stress during the transition from preclinical to clinical training., (© 2022 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd.)