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Objectives Development of a new ultrasound (US) synovitis score (SONography in Arthritis and Rheumatism (SONAR)-7) with evaluation of its diagnostic performance in a cohort of patients with psoriatic arthritis (PsA) and axial spondyloarthritis (axSpA), in comparison with healthy controls (HC).Methods We included 121 participants: 41 patients with PsA and 39 patients with axSpA, from six Swiss hospitals and 41 HC. All participants underwent a clinical examination of joints (68/66 tender joint count/swollen joint count (SJC)), followed by a detailed musculoskeletal US examination of 22 joints to assess for greyscale (GS) and power Doppler (PD) synovitis in adherence with the definitions established by the Outcome Measures in Rheumatology US working group.Results The ‘SONAR-7’ score incorporated GS and PD lesions across 14 joints, including four metacarpophalangeal joints, four distal interphalangeal joints of the hands, two metatarsophalangeal joints, knees and wrists. It demonstrated an area under the receiver operating characteristic curve of 0.831 (95% CI 0.76 to 0.90), with an excellent specificity (95.1%) and moderate sensitivity (44%), comparable to existing US scores. The inflammatory components of the SONAR-7 correlated significantly with clinical measures of inflammation and disease activity (correlation: 0.37 for SJC-28, 0.33 for Disease Activity in Psoriatic Arthritis and 0.48 for US enthesitis). New bone formation in the same joints was associated with the Health Assessment Questionnaire (p=0.035) and Bath Ankylosing Spondylitis Metrology Index (p=0.016).Conclusion The SONAR-7 score represents a fast and effective US-based tool for the evaluation of inflammatory and structural joint involvement in patients with PsA and axSpA, with high specificity and meaningful associations with clinical and quality-of-life outcomes.