Primary mediastinal B-cell lymphoma (PMBL) is a subtype of diffuse large B-cell lymphoma (DLBCL) that is putatively derived from a thymic B cell. Accounting for up to 10% of cases of DLBCL, this subtype predominantly affects women in the third and fourth decades of life. Its clinical and molecular characteristics are distinct from other subtypes of DLBCL and, in fact, closely resemble those of nodular sclerosing Hodgkin lymphoma (NSHL). Recently, mediastinal lymphomas with features intermediate between PMBL and NSHL, called mediastinal gray-zone lymphomas, have been described. The optimal management of PMBL is controversial, and most standard approaches include a combination of immunochemotherapy and mediastinal radiation. Recently, the recognition that mediastinal radiation is associated with significant long-term toxicities has led to the development of novel approaches for PMBL that have shown excellent efficacy and challenge the need for routine mediastinal radiation.