The increasing penetration of digital technologies has transformed the marketing environment of small and medium-sized enterprises (SMEs), creating opportunities for firms to reach customers, strengthen brand visibility, improve customer acquisition, and enhance profitability at comparatively lower costs than conventional marketing channels. This study examined the effect of online marketing on the profitability of SMEs in Enugu State, Nigeria, with particular attention to the mediating role of marketing effectiveness. Specifically, the study investigated the effects of social media marketing, search engine marketing, and email/content marketing on SME profitability. The Technology-Organization-Environment (TOE) framework provided the conceptual foundation for explaining variations in the adoption and utilization of online marketing technologies. A quantitative cross-sectional research design was adopted. The study covered 550 registered SMEs operating predominantly in trade, services, and manufacturing across Enugu North, Enugu South, Nsukka, Udi, Ezeagu, and other local government areas of Enugu State. Online marketing was operationalized through social media marketing, search engine marketing, and email/content marketing, while marketing effectiveness was assessed using customer acquisition and brand visibility. Profitability was measured through monthly sales, net profit margin, return on investment from online marketing, and a composite Business Profitability Index. Descriptive statistics, Pearson correlation, multiple regression, and mediation analysis using PROCESS Model 4 with 5,000 bootstrap samples were specified for data analysis. The reported analytical results indicate that overall online marketing has a positive and statistically significant relationship with business profitability (r = .52, p < .01), while marketing effectiveness demonstrates an even stronger positive association with profitability (r = .68, p < .01). SMEs that utilized an integrated combination of social media, search engine, and email/content marketing also recorded approximately 35% higher net profit margins than firms with less diversified online marketing practices. The mediation model further supports the proposition that online marketing enhances profitability partly by increasing customer acquisition and improving brand visibility. The study concludes that online marketing constitutes an important strategic capability for improving the financial performance and competitiveness of SMEs in Enugu State. It recommends that SME owners adopt integrated digital marketing strategies, strengthen digital skills, consistently measure online marketing returns, and utilize customer and platform analytics to improve marketing effectiveness and long-term profitability.