Osteoarthritis (OA) is common and its prevalence is increased in military service members. In a phase 3 randomized controlled trial (NCT02357459), a single intra-articular injection of an extended-release formulation of triamcinolone acetonide (TA-ER) in participants with unilateral or bilateral knee OA demonstrated substantial improvement in pain and symptoms. Bilateral knee pain has emerged as a confounding factor in clinical trials when evaluating the effect of a single intra-articular injection. Furthermore, unilateral disease is frequently first to emerge in active military personnel secondary to prior traumatic joint injury. In this post hoc analysis, we assessed efficacy and safety of TA-ER in a subgroup of participants with unilateral knee OA. Participants ≥ 40 years of age with symptomatic knee OA were randomized to a single intra-articular injection of TA-ER 32 mg, TA crystalline suspension (TAcs) 40 mg, or saline-placebo. Average daily pain (ADP)-intensity and rescue medication use were collected at each of weeks 1-24 postinjection; Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC)-A (pain), WOMAC-B (stiffness), WOMAC-C (function), and Knee Injury and Osteoarthritis Outcome Score Quality of Life (KOOS-QoL) were collected at weeks 4, 8, 12, 16, 20, and 24 postinjection. Adverse events (AEs) were assessed throughout the study. Participants with unilateral knee OA were selected for this analysis. Of 170 participants with unilateral OA (TA-ER, N = 51; saline-placebo, N = 60; TAcs, N = 59), 42% were male and 89% were white. TA-ER significantly (p < 0.05) improved ADP-intensity vs. saline-placebo (weeks 1-24) and TAcs (weeks 4-21). TA-ER significantly (p < 0.05) improved WOMAC-A vs. saline-placebo (all time points) and TAcs (weeks 4, 8, 12, 24). Consistent outcomes were observed for rescue medication, WOMAC-B, WOMAC-C, and KOOS-QoL. AEs were similar in frequency/type across treatments. TA-ER provided 5-6 months' pain relief that consistently exceeded saline-placebo and TAcs, suggesting that TA-ER injected intra-articularly into the affected knee may be an effective non-opioid treatment option. Although the participants included in this analysis did not fully represent the diverse demographics of active service members, the substantial unmet medical need in the military population suggests that TA-ER may be an important treatment option; additional studies of TA-ER in active military patients are needed. ClinicalTrials.gov NCT02357459. Flexion Therapeutics, Inc. Plain language summary available for this article.