Background-This study investigated the relationship between space available for eruption of impacted mandibular third molars (retromolar space) and arch length tooth size discrepancy (ALTSD) in young adults. The findings provide valuable insights into the aetiology of third molar impaction and its association with dental crowding. Aim-The aim of this study was to evaluate the correlation between retromolar space (RMS) and arch length tooth size discrepancy (ALTSD) in young adults. Understanding this relationship can inform orthodontic treatment planning and prevent complications. Materials and Methods: The study comprised of 100 patients. Arch length tooth size deficiency scoring was calculated from study models, space availability for third molars- retromolar space was measured from OPG, Mandible length and skeletal malocclusion was identified from Cephalogram. Results-The data showed a significant negative correlation between arch length tooth size deficiency and retromolar space with a correlation coefficient of 0.286 (P=0.04).The scatter plot defines the negative correlation of molar and difference, as the difference increases the space for molar is decreasing with a correlation value of - 0.286, which is statistically significant (p=0.004). Conclusion: The study documents a significant correlation between RMS for third molars and arch length tooth size discrepancy in young adults. These findings suggest that ALTSD is a contributing factor to third molar impaction.