This multi-country evaluation assessed the performance of syndromic surveillance systems for early detection of infectious disease outbreaks across 15 national public health institutes from 2018 to 2023. Analyzing 423 verified outbreaks across six syndromic categories, we found that syndromic surveillance detected outbreaks a median of 4.2 days earlier than traditional laboratory-based surveillance. System sensitivity varied significantly by syndrome type, data source, and algorithmic approach, with respiratory and gastrointestinal syndromes showing the highest detection performance. The findings provide evidence-based guidance for optimizing syndromic surveillance systems in diverse public health contexts.