The purpose of the study was to evaluate the clinical endodontic diagnostic skills amongst undergraduate dental students at pre-clinical and clinical levels at Cardiff University School of Dentistry. An online questionnaire containing eight questions about endodontic diagnosis and hypothetical clinical scenarios was sent to all year 3rd, 4th, and 5th-year undergraduate dental students who were divided into G1, G2, and G3 groups. The data were analysed descriptively and reported in percentages. Around 121 students out of 226 responded to the questionnaire with a response rate of 53.5%. The overall correct response from G1 (3rd year) was 31.6% to 65.8%, G2 (4th year) was 73% to 93%, and G3 (5th year) was 73.2% to 92.7%. The study concludes that the 4th and 5th-year undergraduate dental students’ responses to the hypothetical clinical scenarios were higher than the 3rd-year students. However, regarding questions about the endodontic diagnosis, the percentages of correct answers were similar among all the 3rd, 4th, and 5th-year students. Therefore, further studies assessing endodontic diagnostic skills amongst the same cohort of students during their progression in the undergraduate course are recommended.