To assess hard as well as soft peri-implant tissues within cases having two lost adjacent anterior teeth treated through placing either two implants with two separate crowns or only an implant along with a crown with a cantilever, and evaluating the effect of polyetheretherketone (PEEK) restoration on cantilever design up to 18 months after functional loading.Twenty-seven participants (15 males and 12 females; mean age, 38.6 years; range 20-50 years) with missing two adjacent anterior teeth were treated with implant system (Flotecno implant system, Italy). In the first group (implant-implant metal ceramic group), we treated nine participants utilizing two adjacent implants with two separate single metal ceramic crowns. In the second group (implant-cantilever metal ceramic group), we treated nine participants by placing single implant with cantilever metal ceramic fixed dental prosthesis (FDP). In the third group (implant-cantilever PEEK group), we treated nine cases utilizing single implant with a cantilever PEEK FDP framework. Clinical and radiographic examinations were recorded. Marginal bone level, implant stability, and prosthetic complications were assessed during an 18-month follow-up period.Marginal bone loss (MBL) exhibited similar measurements among all groups. The clinical outcomes did not address significant variance among all groups as regards implant stability within the period of follow-up. We also observed minor prosthetic complications. Participants were very satisfied within all groups.Based on the limitations of our research, utilizing cantilever extensions has no influence on MBL as well as implant stability.Mean marginal bone level exhibited a significant rise from baseline to 18 months for all groups, however, still within the clinically accepted range. Regarding implant stability, no significant variance was observed among all groups for 18 months. The cantilever FDP design facilitated prosthesis fabrication among those having laterals of narrow diameters. Further research is required to investigate such a particular concern due to a limited sample size in our research. How to cite this article: Hussain MR, Shrif MM, Othman HI, et al. Comparative Study between Two Adjacent Implants Supported Crowns and One Implant Supported Cantilever Fixed Dental Prosthesis: An In Vivo Study. J Contemp Dent Pract 2024;25(10):983-991.