This study evaluated the efficacy of green lipped mussel (GLM), added to a complete dry diet, for alleviating clinical signs of arthritis in dogs. A double-blind longitudinal study design was used with 31 mixed-breed dogs exhibiting varying degrees of arthritis. Each dog was evaluated by a veterinarian and joints were individually scored for degree of pain, swelling, crepitus, and reduction in range of movement. Summation of all scores for an individual dog comprised its total arthritis score. At baseline, dogs were randomly allocated to control and test groups. Both groups were fed the same base dry diet, to which 0.3% GLM powder was added for dogs in the test group. The change in total arthritis score by the end of 6 weeks showed there was significant improvement (P < .05) in the test group versus the control group. Significant improvements were also observed in joint pain and swelling scores in the test group. Changes in joint crepitus and range of joint movement were not significantly different between the test and control groups. These findings provide strong evidence that GLM incorporated into a complete dry diet can help alleviate arthritis symptoms in dogs.