Lambda Sigma (ΛΣ) is an American college honor society for second-year students. Originally named the Society of Cwens, the society was established at the University of Pittsburgh in Fall 1922 as a women's honors society and became a national organization with the 1925 foundation of chapters at Miami University and the University of Missouri. The society is "dedicated to the purpose of fostering leadership, scholarship, fellowship, and the spirit of service among college students, and to promoting the interests of the college or university in every possible way".

In the United States, an honor society is an organization that recognizes individuals who excel in various domains such as academics, leadership, and other personal achievements, not all of which are based on ranking systems. These societies acknowledge excellence among peers in diverse fields and circumstances. The Order of the Arrow, for example, is the National Honor Society of the Boy Scouts of America. While the term commonly refers to scholastic honor societies, which primarily acknowledge students who excel academically or as leaders among their peers, it also applies to other types of societies.