Objective This study aimed to find the relation of the MS co‐existence with the decayed, missing (Extracted) and filled teeth (DMFT) score and the prevalence of Streptococcus mutans serotypes in the Iranian population. Methods In this cross‐sectional research conducted in 2018, volunteers aged 10–60< years were measured by population selection criteria. PCR technique was used to screen MS serotypes in the homogenized saliva and plaque samples. Results 499 subjects met the selection criteria of the study population. Out of 499 samples, 448 samples were finalized for serotype determination and DMFT relation evaluation. From 448, 348 (77.6%) samples harboured only S. mutans, 44 (9.8%) had both S. mutans and S. sobrinus, 6 (1.3%) were positive for S. sobrinus alone, and 94 (20.9%) were free of both specious. The mean DMFT score in people with S. mutans (6.7) was higher than S. mutans negative (4.6) participants (p < 0.05). In people with both S. mutans and S. sobrinus, the mean DMFT did not change significantly. The frequency of c, e, f and k serotypes was 47.7, 22.7, 27.5 and 22.1%, respectively. The mean DMFT score in participants with serotype e was significantly higher than others (p < .05). Conclusions People can acquire different S. mutans serotypes over a lifetime. The high prevalence rate of serotype k, a systemic S. mutans serotype, calls worldwide studies on the prevalence of serotype k strains.