Osteoarthritis (OA) is a common chronic disorder among elderly people that affects joints such as the knee and hip in particular. The objective of the current study was to examine the efficacy of an intervention based on a theory of planned behavior (TPB) in improving health-related quality of life in middle-age and older adults with this condition. One hundred twenty patients diagnosed with knee/hip OA were recruited from a general hospital. Measures administered at baseline were the SF-12, EuroQol (EQ-5D), Osteoarthritis Knee and Hip Quality of Life (OAKHQoL), and TPB questionnaire. Also assessed were body mass index (BMI), Kellgren-Lawrence Scale, six-minute walk test (SMWT), muscle strength, range of motion (ROM), and joint tenderness and swelling. Participants were randomly assigned to either the intervention (n = 60) or the control group (n = 60). The intervention group received an educational program based on TPB that was administered over 1 month. The control group did not receive this treatment. Three months after the intervention, both groups were reevaluated and comparisons made. Compared to the no-treatment control group, those in the intervention group scored higher on HRQoL, both general and specific, at 3-month follow-up (p < 0.01). The only exception was the vitality domain. Significant differences were also found on the TPB questionnaire, the SMWT, and muscle strength in the expected direction (p < 0.05). While those in the intervention group improved significantly on all clinical measures from pre-test to post-test, those in the control group showed improvement only on BMI and joint tenderness. This TPB-based intervention was found to be efficacious in improving HRQoL and several clinical parameters in patients with knee/hip OA. Studies are needed to examine the effectiveness of this intervention in patients with other chronic medical conditions.