This study explores how small and medium-sized enterprises (SMEs) in Zambia and Zimbabwe use digital marketing to support internationalisation under conditions of resource constraint and institutional fragility. While digital technologies have lowered barriers to global market entry, existing literature provides limited theoretical and empirical insight into how SMEs in resource-constrained African contexts strategically integrate digital marketing into export activities. Adopting a grounded theory approach, this study inductively develops theory from qualitative data collected from 10 SMEs engaged in digital cross-border trade. Data were analysed using iterative open, axial, and selective coding procedures. Four core categories emerged: Network-Enabled Market Access, Experiential Learning Under Constraint, Adaptive Digital Workarounds, and Hybrid Internationalisation Pathways. The study develops an Adaptive Digital Internationalisation Framework, demonstrating that SMEs achieve export competitiveness through adaptive capacity, relational networks, and digital improvisation rather than resource accumulation alone. This extends the Resource-Based View, Uppsala Model, and Born Global perspectives by highlighting the role of social embeddedness and contextual adaptation in shaping internationalisation under constraint. Practical implications are offered for SMEs, policymakers and development institutions seeking to strengthen digital export participation.