OBJECTIVE: Our objective was to develop and validate a composite disease flare definition for juvenile spondyloarthritis that would closely approximate the clinical decision made to reinitiate/not reinitiate systemic therapy after therapy de-escalation. METHODS: Retrospective chart reviews of children with spondyloarthritis who underwent systemic therapy de-escalation of biologic or conventional disease-modifying antirheumatic drugs (bDMARDs; cDMARDs) were used to develop and validate the flare outcome. Independent cohorts for development (1 center) and validation (4 centers) were collected from large tertiary healthcare systems. Core measure thresholds and candidate disease flare outcomes were assessed using sensitivity, specificity, positive (PPV) and negative predictive values (NPV), and area under the receiver operating characteristic (AUROC) curve with physician assessment of "active disease" plus re-initiation of standard dose of systemic therapy as the reference standard. RESULTS: Of the candidate definitions, clinically meaningful worsening in ≥3 of the following five core measures performed best: caregiver/patient assessment of well-being, physician assessment of disease activity, caregiver/patient assessment of pain, physical function, and active joint count. AUROC was 0.91, PPV 87.5%, NPV 98.1%, sensitivity 82.4%, and specificity 98.7%. Cronbach's α was 0.81, signifying internal consistency and factor analysis demonstrated the outcome measured one construct. "JSpAflare" had face validity according to 21 surveyed pediatric rheumatologists. JSpAflare had AUROC 0.85, PPV 92.3%, and NPV 96.8% in the validation cohort. CONCLUSIONS: There is initial support for the validity of JSpAflare as a tool to identify disease flare in juvenile spondyloarthritis patients de-escalating therapy and is potentially applicable in clinical practice, observational studies, and therapeutic trials.