Information on the association of self-reported knee instability with clinical outcomes after Total Knee Arthroplasty (TKA) and 1 year follow-up is scarce. The aims were to determine (i) the course and prevalence of self-reported knee instability before and 1 year after TKA and (ii) the associations of preoperative, postoperative, and retained self-reported knee instability with pain, activity limitations, and quality of life (QoL) in patients with knee osteoarthritis. Patients undergoing primary TKA, selected from the Longitudinal Leiden Orthopaedics and Outcomes of OsteoArthritis Study, had their knee instability measured using a questionnaire. The Knee injury and Osteoarthritis Outcome Score pain, activity limitations, and QoL subscales were administered before and 1 year after surgery. Multivariable regression analyses were performed to examine associations between knee instability, pain, activity limitations, and QoL, adjusted for covariates (age, gender, comorbidities, and radiographic severity). Of the 908 included patients, 649 (71%) and 187 (21%) reported knee instability before and following TKA, respectively. Of the patients with preoperative knee instability, this perception was retained in 165 (25%) cases. Knee instability was preoperatively associated with pain (B −9.6; 95%CI: −12.4 to −6.7), activity limitations (B −7.5; 95%CI: −10.2 to −4.8), and QoL (B −4.7; 95%CI: −7.0 to −2.4) and postoperatively with pain (B −15.0; 95%CI: −18.5 to −11.6), activity limitations (B −15.1; 95%CI: −18.4 to −11.8), and QoL (B −18.7; 95%CI: −22.3 to −15.3). Retained knee instability was associated with postoperative pain (B −15.1; 95%CI: −18.9 to −11.2), activity limitations (B −14.1; 95%CI: −17.8 to −10.4), and QoL (B −18.0; 95%CI: −21.7 to −14.3). In conclusion, in clinical care, self-reported knee instability is retained postoperatively in 25% of the patients. Retained knee instability is associated with more pain, activity limitations, and poorer QoL postoperatively. © 2018 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:2671–2678, 2018.