BACKGROUND: Yoga is a mind-body exercise typically done in groups in person, but this delivery method can be inconvenient, inaccessible, and costly. Effective online programs may increase access to exercise for knee osteoarthritis. OBJECTIVE: To evaluate the effectiveness of an unsupervised 12-week online yoga program. DESIGN: Two-group superiority randomized trial. (Australian New Zealand Clinical Trials Registry: ACTRN12620000012976). SETTING: Community. PARTICIPANTS: 212 adults with symptomatic knee osteoarthritis. INTERVENTION: Both groups received online osteoarthritis information (control). The yoga group also received access to an unsupervised online yoga program delivered via prerecorded videos over 12 weeks (1 video per week, with each session to be performed 3 times per week), with optional continuation thereafter. MEASUREMENTS: Primary outcomes were changes in knee pain during walking (0 to 10 on a numerical rating scale) and physical function (0 to 68 on the Western Ontario and McMaster Universities Osteoarthritis Index) at 12 weeks (primary time point) and 24 weeks, analyzed using mixed-effects linear regression models. Secondary outcomes were self-reported overall knee pain, stiffness, depression, anxiety, stress, global change, quality of life, self-efficacy, fear of movement, and balance confidence. Adverse events were also collected. RESULTS: A total of 195 (92%) and 189 (89%) participants provided 12- and 24-week primary outcomes, respectively. Compared with control at 12 weeks, yoga improved function (between-group mean difference in change, -4.0 [95% CI, -6.8 to -1.3]) but not knee pain during walking (between-group mean difference in change, -0.6 [CI, -1.2 to 0.1]), with more yoga participants than control participants achieving the minimal clinically important difference (MCID) for both outcomes. At 12 weeks, knee stiffness, quality of life, and arthritis self-efficacy improved more with yoga than the control intervention. Benefits were not maintained at 24 weeks. Adverse events were minor. LIMITATION: Participants were unblinded. CONCLUSION: Compared with online education, an unsupervised online yoga program improved physical function but not knee pain at 12 weeks in people with knee osteoarthritis, although the improvement did not reach the MCID and was not sustained at 24 weeks. PRIMARY FUNDING SOURCE: National Health and Medical Research Council and Centres of Research Excellence.