This study examines how advertising expenditure volatility affects firm value and whether ESG performance moderates this relationship. Unlike prior marketing–finance research emphasizing advertising levels as investments in brand capital, it considers volatility as market-facing investment uncertainty that may signal a firm’s strategic commitment and managerial discipline. Using KOSPI and KOSDAQ firms from 2012 to 2023, advertising volatility is measured as the rolling standard deviation of advertising intensity, while next-period Tobin’s Q proxies for firm value. ESG ratings and pillar scores are obtained from the Korea Corporate Governance Service. Results indicate that the unconditional relationship between advertising volatility and firm value is not consistently significant, implying that volatility is not inherently value-destroying. However, its valuation effect depends on ESG performance. Advertising volatility is negatively associated with future firm value among low-ESG firms, but this association is significantly weaker among high-ESG firms. Thus, ESG performance may provide credibility and reputational capital, encouraging investors to interpret volatile advertising as adaptive flexibility rather than strategic inconsistency or short-termism. Additional analyses show that this moderating effect is driven mainly by the Social and Governance dimensions and remains robust across alternative volatility windows. The study highlights advertising stability and ESG’s role as a credibility- based interpretive mechanism.