We describe the clinical and histopathological results of plaque psoriasis in eleven adult patients with knee osteoarthritis and long-standing, moderate to severe psoriasis resistant to conventional therapy treated with chondroitin sulfate. Patients received 800 mg per day of chondroitin sulfate for 2 months. Skin biopsies were obtained before and after treatment. All patients but one presented a dramatic improvement of the condition of the skin, with a reduction of swelling, redness, flaking, and itching (clearance of psoriasis in one patient), increase in the hydration and softening of the skin, and amelioration of scaling. Histopathologically, there was a statistically significant decrease in epidermal thickness, a decrease in the thickness between the stratum basale and the stratum granulosum, a significant improvement of the degree of psoriasis activity, and a decrease in the number of keratinocytes stained with Ki-67. The confirmation of these serendipitous findings in controlled prospective studies could represent an important advance in the therapeutic armamentarium for patients with psoriasis given the excellent safety profile of chondroitin sulfate.