Despite the growing adoption of sustainable practices by luxury companies, the luxury industry encounters a unique paradox; while consumers expect luxury brands to act responsibly, the very essence of luxury is often seen as conflicting with sustainability. Drawing on a sample of 323 luxury consumers in Greece and employing a self-administered questionnaire, the present chapter explores the impact of Sustainable Marketing Activities (SMAs) on consumer behavior in the luxury market, focusing on their impact on brand trust, engagement, and defense. The findings reveal that economic, social, and cultural SMAs significantly strengthen brand trust and engagement, whereas the influence of environmental SMAs remains limited. However, environmental SMAs positively influence brand defense, driven by consumers’ desire for social signaling. A three-path mediation model demonstrates that SMAs impact brand defense through brand trust and engagement. The chapter underscores the necessity of harmonizing sustainability with luxury values and provides practical insights for luxury brands to improve consumer relationships, while navigating the sustainability-luxury paradox.