This study investigated the effect of social media marketing on the business performance of small and medium enterprises (SMEs) in Kampala, Uganda. As social media platforms including Facebook, WhatsApp, Twitter/X, Instagram, and TikTok had rapidly penetrated Uganda's digital landscape, SMEs had increasingly experimented with these platforms as cost-effective marketing channels. However, the empirical evidence on whether and how social media marketing translated into improved business performance remained thin and contested, particularly in the Ugandan context. Using a cross-sectional survey of 298 SME owners and managers in Kampala, the study examined the effects of social media content marketing, social media advertising, customer engagement, and brand awareness building on SME business performance measured through sales growth, customer acquisition, and market share expansion. The results demonstrated that social media marketing had a statistically significant and positive effect on SME business performance, with customer engagement and brand awareness emerging as the most influential dimensions.