Simone de Beauvoir (1908–1986) was a philosopher, writer, and critical theorist who played a central role in shaping postwar existentialism. Her work addresses themes such as freedom, subjectivity, gender, oppression, and ethics and had a significant impact on feminist thought by challenging the social constructions of womanhood, most notably in her seminal 1949 work The Second Sex. This chapter explores Beauvoir’s biography and bibliography, her main philosophical influences, and her impact on theoretical and philosophical psychology, particularly in relation to freedom, the ethics of ambiguity, and embodied subjectivity. Beauvoir’s philosophy continues to inform contemporary debates on gender, ethics, and psychology, offering a vital critique of oppressive structures and the supposed neutrality of science.