Background: The need for certain amount of keratinized tissue around dental implants for maintaining optimal peri-implant tissue health has been discussed for many years. This systematic review aims to investigate the effect of keratinized tissue on various peri-implant health-related parameters. Materials and methods: An electronic search of Pubmed, Scopus and Web of Science for relevant articles were performed. Human studies with data on the relationship between the amount of existing keratinized tissue around dental implants supporting fixed prosthesis and various peri-implant parameters, with a follow-up period of at least 6 months, were included. Results: Nine studies, seven cross-sectional and two longitudinal, were included. Weighted mean difference (WMD) and confidence interval (CI) were calculated with meta-analyses for each clinical parameter. The results showed statistically significant differences in plaque index (PI) and modified plaque index (mPI), gingival index (GI), recession, and bone loss, all favoring implants with wide keratinized tissue. However, comparisons of other parameters (bleeding on probing, modified bleeding index (mBI), and probing depth, did not reach statistically significant differences. The result of heterogeneity test showed only probing depth had a low degree of heterogeneity among analyzed studies. Limitations of the present review include limited number of selected studies (n = 9), existence of heterogeneity and publication bias, and only English-written articles searched. Conclusion: Based on current available evidence, a lack of adequate keratinized tissue around dental implants is associated with more plaque accumulation, tissue inflammation, recession, and bone loss.