Abstract: Acts of extraordinary, costly altruism in which significant risks or costs are assumed to benefit strangers have long represented a motivational puzzle. But the features that consistently distinguish individuals who engage in such acts have not been identified. We conducted the first assessment of six groups of real-world extraordinary altruists who had performed costly or risky and normatively very rare (<.00005% per capita) altruistic acts: heroic rescues, non-directed and directed kidney donations, liver donations, marrow or hematopoietic stem cell donations, and humanitarian aid work. The features that best distinguished altruists from controls were traits and decision-making patterns indicating unusually high valuation of others’ outcomes: high Honesty-Humility, reduced Social Discounting, and reduced Personal Distress—results that two independent samples of adults failed to predict. Findings suggest that theories regarding self-focused motivations for altruism (e.g., self-enhancing reciprocity, reputation enhancement) alone are insufficient explanations for acts of real-world self-sacrifice.