Knee osteoarthritis is prevalent in older adults worldwide. Quality of life was negatively affected by the COVID-19 pandemic. This study was designed to examine osteoarthritis severity and health-related quality of life (QOL) in older adults with knee osteoarthritis before total knee replacement during the COVID-19 pandemic and to identify the related predictors of QOL. This cross-sectional correlation study involved convenience sampling in the orthopedic ward of a regional teaching hospital in central Taiwan from June 2020 to June 2021 using the Western Ontario and McMaster Universities Arthritis Index and the SF-36v2 Health Survey. The data were analyzed using Pearson's correlation coefficient analysis, independent samples t test, and one-way analysis of variance to determine correlations among demographic variables, osteoarthritis severity, and QOL. The predictors of QOL were examined using stepwise multiple linear regression analysis. A total of 60 older adults diagnosed with knee osteoarthritis were sampled. The average age was 70 years and the average osteoarthritis severity score was 70.45. Being male, having comorbidities, and having a relatively high level of monthly disposable income were associated with poorer QOL. Moreover, more severe knee pain, stiffness, and physical dysfunction were associated with better psychological QOL. During the COVID-19 pandemic, the severity of knee osteoarthritis affects preoperative quality of life in older adults. Clinicians should detect signs of pain and physical dysfunction in these patients in advance and intervene in a timely manner to improve their QOL before surgery.