Background: It is unclear if single implant vs. two implant-supported mandibular overdentures have similar clinical outcomes especially pertaining to implant survival, marginal bone loss (MBL) and prosthetic complications. A systematic review and metaanalysis of randomized controlled trials (RCTs) was conducted to examine this clinical query. Methods: Main scientific databases (PubMed, Embase, Scopus, Web of Science, and Cochrane Library) were searched from inception until October 6th, 2024, for RCTs comparing single vs two implants for mandibular overdentures and reporting implant failures, MBL and prosthetic complications. Results: Eleven RCTs were included. Meta-analysis showed a tendency of reduced implant failure rates with single implants at 1 year (OR: 0.33 95% CI: 0.10, 1.15 I2=0%) and 5 years (OR: 0.11 95% CI: 0.01, 2.16), but the results were not statistically significant. However, pooled subgroup analysis of studies with a follow-up period of 2-3 years indicated a significantly reduced implant failure rate with one single as compared to two implants (OR: 0.12 95% CI: 0.03, 0.54 I2=0%). No significant differences in implant failure rate were observed between the two groups based on implant loading protocols. Meta-analysis also showed that there were no significant differences in the risk of MBL between single and two implant groups (MD: -0.15 95% CI: -0.31, 0.01 I2=43%). Pooled analysis also showed that the risk of overdenture fracture, relining, and metal housing re-attachment did not differ significantly between single and two implant groups. Conclusions: Evidence from a limited number of RCTs, mainly with small sample sizes, indicates that single implant-supported mandibular overdentures may have a tendency of lower implant failures as compared to two implant-supported mandibular overdentures. The risk of MBL and prosthetic complications may not differ with either treatment modality.