This study examines the effect of the United States’ Reciprocal Tariff Announcement of 2 April 2025 on stock markets, in the Gulf Cooperation Council (GCC) and Association of Southeast Asian Nations (ASEAN) markets using event study framework. The results reveal that the tariff announcement caused a substantial negative information shock, triggering adverse abnormal returns in both regions. ASEAN markets experienced greater losses, implying their greater dependence on global manufacturing networks and export-driven supply chains. On the contrary, GCC markets adjusted more slowly, a pattern aligned with market structures that are relatively less liquid and more fragmented. Meanwhile, sector-level analysis reveals that industries with a strong reliance on trade, including technology, industrials, transportation, and energy-related sectors, incurred the most enduring losses, whilst domestic sectors remained relatively resilient. The observation of abnormal returns persisting over several days indicates a slower adjustment of prices, especially in less liquid markets. These findings extend the literature on trade shocks by demonstrating that tariff announcements generate heterogeneous and enduring market responses across emerging economies, providing insights for investors and policymakers seeking to mitigate vulnerability in market structures. JEL: G1, G14