Background: Treat-to-target strategies have improved outcomes in single-organ diseases with simple clinical or laboratory endpoints. A lack of validated endpoints has prevented adoption of treat to target for complex multiorgan conditions, such as systemic lupus erythematosus (SLE). We report the first prospective study undertaken to specifically validate a treat-to-target endpoint for SLE. Methods: In this prospective cohort study, patients aged 18 years or older with SLE were recruited from 13 centres in eight countries and followed prospectively. Patients with at least two visits over the study period no more than 6 months apart were included in the longitudinal analysis. Patients with no visits in the final year of the study were censored from their last visit. Attainment of the lupus low disease activity state (LLDAS) was assessed at each visit. The primary outcome measure was accrual of irreversible end-organ damage, defined as at least a 1-point increase in the Systemic Lupus International Collaborating Clinics/American College of Rheumatology Damage Index. We used time-dependent hazard regression models and generalised linear models to measure the association between LLDAS (attainment at any timepoint, cumulative time in LLDAS, and sustained LLDAS) with accrual of irreversible end-organ damage or flare (key secondary outcome). This study is registered with ClinicalTrials.gov, NCT03138941. Findings: Between May 1, 2013, and Dec 31, 2016, 1707 patients were recruited and followed for a mean of 2·2 years (SD 0·9), totalling 12 689 visits. Attainment of LLDAS at any timepoint was associated with reduction in damage accrual (hazard ratio 0·59, 0·45–0·76; p<0·0001) and subsequent flare (0·65, 95% CI 0·56–0·75; p<0·0001). Cumulative time in LLDAS was associated with improved outcomes: compared with patients with less than 50% of observed time in LLDAS, those with at least 50% of observed time in LLDAS had reduced risk of damage accrual (0·54, 0·42–0·70; p<0·0001) and flare (0·41, 0·35–0·48; p<0·0001). Similarly, increased durations of sustained LLDAS were associated with incremental reductions in the risk of damage accrual. The association of LLDAS with reduced damage accrual was observed regardless of pre-existing damage or disease activity at study entry. Interpretation: LLDAS attainment is associated with significant protection against flare and damage accrual in SLE. These findings validate LLDAS as an endpoint for clinical studies in SLE. Funding: The Asia Pacific Lupus Collaboration received project support grants from UCB Pharma, GlaxoSmithKline, Janssen, Bristol-Myers Squibb, and AstraZeneca.