When romantic partners sacrifice their own self-interest to benefit the relationship, the sacrificer or recipient may—for various reasons—be biased in how they perceive the costs that the sacrificer incurs. In Study 1, romantic couples (N = 125) rated their own and their partner’s costs after a conversation about a sacrifice in the laboratory, followed by extensive experience sampling in their natural environment. In Study 2, a preregistered experiment, individuals (N = 775) imagined a scenario in which they, their partner, or an unknown person sacrificed and rated the associated costs and benefits. Both studies demonstrated a consistent discrepancy between perceptions of own and partner sacrifice, driven primarily by people underestimating their own sacrifice costs and overestimating the benefits (Study 2). Results across studies showed that this underestimation bias helps people to feel better and feel more satisfied in the relationship when giving up their own goals and preferences for the relationship.