Background The indications for septoplasty are practice-based, rather than evidence-based. In addition, internationally accepted guidelines for the management of nasal obstruction associated with nasal septal deviation are lacking. Objective The objective was to determine the clinical effectiveness and cost-effectiveness of septoplasty, with or without turbinate reduction, compared with medical management, in the management of nasal obstruction associated with a deviated nasal septum. Design This was a multicentre randomised controlled trial comparing septoplasty, with or without turbinate reduction, with defined medical management; it incorporated a mixed-methods process evaluation and an economic evaluation. Setting The trial was set in 17 NHS secondary care hospitals in the UK. Participants A total of 378 eligible participants aged > 18 years were recruited. Interventions Participants were randomised on a 1: 1 basis and stratified by baseline severity and gender to either (1) septoplasty, with or without turbinate surgery (n = 188) or (2) medical management with intranasal steroid spray and saline spray (n = 190). Main outcome measures The primary outcome was the Sino-nasal Outcome Test-22 items score at 6 months (patient-reported outcome). The secondary outcomes were as follows: patient-reported outcomes – Nasal Obstruction Symptom Evaluation score at 6 and 12 months, Sino-nasal Outcome Test-22 items subscales at 12 months, Double Ordinal Airway Subjective Scale at 6 and 12 months, the Short Form questionnaire-36 items and costs; objective measurements – peak nasal inspiratory flow and rhinospirometry. The number of adverse events experienced was also recorded. A within-trial economic evaluation from an NHS and Personal Social Services perspective estimated the incremental cost per (1) improvement (of ≥ 9 points) in Sino-nasal Outcome Test-22 items score, (2) adverse event avoided and (3) quality-adjusted life-year gained at 12 months. An economic model estimated the incremental cost per quality-adjusted life-year gained at 24 and 36 months. A mixed-methods process evaluation was undertaken to understand/address recruitment issues and examine the acceptability of trial processes and treatment arms. Results At the 6-month time point, 307 participants provided primary outcome data (septoplasty, n = 152; medical management, n = 155). An intention-to-treat analysis revealed a greater and more sustained improvement in the primary outcome measure in the surgical arm. The 6-month mean Sino-nasal Outcome Test-22 items scores were −20.0 points lower (better) for participants randomised to septoplasty than for those randomised to medical management [the score for the septoplasty arm was 19.9 and the score for the medical management arm was 39.5 (95% confidence interval −23.6 to −16.4; p < 0.0001)]. This was confirmed by sensitivity analyses and through the analysis of secondary outcomes. Outcomes were statistically significantly related to baseline severity, but not to gender or turbinate reduction. In the surgical and medical management arms, 132 and 95 adverse events occurred, respectively; 14 serious adverse events occurred in the surgical arm and nine in the medical management arm. On average, septoplasty was more costly and more effective in improving Sino-nasal Outcome Test-22 items scores and quality-adjusted life-years than medical management, but incurred a larger number of adverse events. Septoplasty had a 15% probability of being considered cost-effective at 12 months at a £20,000 willingness-to-pay threshold for an additional quality-adjusted life-year. This probability increased to 99% and 100% at 24 and 36 months, respectively. Limitations COVID-19 had an impact on participant-facing data collection from March 2020. Conclusions Septoplasty, with or without turbinate reduction, is more effective than medical management with a nasal steroid and saline spray. Baseline severity predicts the degree of improvement in symptoms. Septoplasty has a low probability of cost-effectiveness at 12 months, but may be considered cost-effective at 24 months. Future work should focus on developing a septoplasty patient decision aid. Trial registration This trial is registered as ISRCTN16168569 and EudraCT 2017-000893-12. Funding This award was funded by the National Institute for Health and Care Research (NIHR) Health Technology Assessment programme (NIHR award ref: 14/226/07) and is published in full in Health Technology Assessment; Vol. 28, No. 10. See the NIHR Funding and Awards website for further award information.