The global cosmetic retail industry faces increasing pressure from consumers and regulators to demonstrate genuine commitment to environmental sustainability and transparency1. This environment necessitates effective communication at the point of sale to prevent the perception of greenwashing2. This study aims to investigate the influence of sales employees' specific Green Knowledge, categorized into Green Packaging Knowledge and Green Ingredient Knowledge, on their Green Service Behavior (GSB) in the Thai cosmetic retail sector3. Employing a quantitative survey design, data was collected from a sample of $N=246$ frontline sales employees using a non-probability quota sampling method4. Analysis using Multiple Linear Regression (MLR) revealed that both Green Packaging Knowledge and Green Ingredient Knowledge are significant positive predictors of Green Service Behavior5. The findings, framed within the Theory of Planned Behavior, highlight that specialized knowledge enhances the employee's Perceived Behavioral Control, translating directly into high-quality GSB6. The research provides strong empirical justification for strategic investment in targeted, evidence-based employee training as a primary mechanism to enhance GSB, solidify customer trust, and effectively mitigate the reputational and regulatory risks associated with greenwashing7777.