Introduction <p>Patient-reported outcome measures are essential tools in clinical decision-making and research. Multi-item scores are time-consuming to collect and evaluate leading to bias due to missing data. This effect is intensified in vulnerable patient groups with reduced mental health, such as patients with adult idiopathic scoliosis (AdIS). The subjective spine value (SSpV), as a single-item value, assesses spinal function with one question: “What is the overall percent value of your spine if a completely normal spine represents 100%?”. The SSpV was previously validated in a variety of specific spinal disorders. To date, no study assessed the SSpV in patients with AdIS. The hypothesis was that the novel single-item score SSpV would correlate with the established Oswestry disability index (ODI), Core Outcome Measures Index for the back (COMI-back), and Scoliosis Research Society Score (SRS-22) questionnaire in patients with AdIS.</p> Materials and methods <p>This cross-sectional study prospectively included 59 patients with AdIS between 04/2022 and 12/2023. The patients completed a questionnaire containing SSpV, ODI, COMI-back, and SRS-22. Spearman's rank correlation coefficient was used to evaluate the correlation between SSpV and the established questionnaires separately. Ceiling and floor effects were evaluated.</p> Results <p>The patients were mainly female (female: n = 46, 78%; male: n = 13, 22%) with a median age of 30&#xa0;years (interquartile range: 23–37&#xa0;years). The Cobb angle ranged from 11° to 89° (mean: 42.3°; SD: 19.0°). SSpV correlated significantly separately with ODI (<i>rs</i> = − 0.487, <i>p</i> ≤ 0.001), COMI-back (<i>rs</i> = − 0.540, <i>p</i> ≤ 0.001), and SRS-22 (<i>rs</i> = 0.626, <i>p</i> ≤ 0.001). Floor and ceiling effects were low (SSpV: 1%–3%; ODI: 2%–1%; COMI-back: 3%–2%; SRS-22: 1%–3%).</p> Conclusions <p>The novel single-item score SSpV validly represents the established multi-item ODI, COMI-back, and SRS-22 in patients with AdIS.</p>