Knee osteoarthritis (OA) has recently been ranked as the 11th highest contributor to global disability. More than 40% of patients use complementary and alternative medicine including supplements containing phytoextracts with anti-inflammatory properties as those from the Boswellia genus. The aim of this meta-analysis was to evaluate the efficacy of phytoextracts from the oleogum resin of the Boswellia genus as supplementation for patients affected by knee OA. Four electronic databases were used for the research and PRISMA statements were followed throughout the study. The following inclusion criteria were used: (a) the subjects of the study were humans with a diagnosis of knee OA reported by medical staff; (b) randomization and the presence of control (placebo, negative or positive control), and (c) outcomes reported with WOMAC and/or visual analog scale (VAS) score. Publication bias was assessed with a funnel plot and through the Egger test. The Jadad scale was used in order to assess the quality of the studies included. The statistical heterogeneity was assessed using I statistics. Results of meta-analysis and subgroup analysis were reported using a forest plot. A total of 13 studies involving 850 (WOMAC) and 1185 (VAS) patients met the inclusion criteria. The meta-analysis did not detect a significant effect of the use of Boswellia extracts between the control and the treatment groups due to the high heterogeneity of the studies (p = 0.0865 for WOMAC) and (p = 0.3966 VAS). However, the subsequent subgroup analysis demonstrated the significant beneficial effect of Boswellia extracts in the treatment of knee OA with respect to a placebo (lower WOMAC score in the treatment groups). This was also confirmed in the meta-regression applied to the WOMAC scores. This is an important finding as people exposed to NSAID-related adverse effects could benefit from the use of Boswellia extracts. However, further high-quality studies are needed to establish the clinical efficacy of extracts from the genus Boswellia.