Objectives: To compare the efficacy and safety of a new fixed dose combination of glucosamine sulfate and chondroitin sulfate capsules (GS/CS) versus the fixed dose combination of glucosamine hydrochloride and chondroitin sulfate (Cosamin DS®) in capsules in patients with osteoarthritis (OA) of the knee. Methods: Multicenter, randomized, double-blind study. Participants with knee OA Kellgren-Lawrence grades 1 to 3 and VAS of symptoms ≥4 cm were randomized to receive GS/CS or Cosamin DS® over 12 weeks. The primary efficacy endpoint was the evaluation of the analgesic efficacy by the investigator. Secondary efficacy endpoints included: joint pain and swelling, investigator efficacy of the medication, and the use of rescue medication. Adverse events and drug tolerability were analyzed. Results: One hundred patients were randomized, and 50 patients were allocated to each group. The analgesic efficacy evaluated by the investigator in the GS/CS group was 88.9, 95%CI: 75.2, 95.8% and in the Cosamin DS® group was 85.4%; 95%CI: 70.1, 93.4%. The mean reduction in the pain intensity was significant in both groups (p < 0.001), with no difference between them. The primary efficacy analysis demonstrated the non-inferiority of the GS/CS group compared with the Cosamin DS® group; the lower limit of the 90% confidence interval (CI) between the two groups (− 8.39%) was higher than the established margin of non-inferiority of − 10.00%. Improvement in other efficacy outcomes was observed, again without differences between groups. Adverse events were similar between groups and both presented good tolerability. Conclusions: The new fixed-dose formulation of GS/CS is effective in treating knee OA, presenting a good safety and tolerability profile. Trial Registration: (https://clinicaltrials.gov/ct2/show/NCT00955552?term=NCT00955552&rank=1; ClinicalTrials.gov; register number NCT00955552; First randomized patient: 08/17/2010).