秘书长（英语：Secretary-general）为政府机关、政党或非营利社团法人（NPO）中经常设置的高端职位之一，通常为机关团体中的幕僚长，承组织首长（委员长、理事长、会长、党主席等）之命总管行政业务，并指挥监督所属职员，并可依状况设置副秘书长以辅佐秘书长职务。类似古代的长史这一官职。若组织有秘书处、室则另有该处首长，或秘书长兼任。

秘书长一职的名称，可能随着各地的民情而有不同的称呼。立法院等民意机关内的党团则称为书记长，各级法院则称为书记官长，民营组织亦可能称总干事。在日本和韩国，则将秘书长称为「干事长」，总书记称为「书记长」，多设于政党和政府机构之中。

Secretary is a title often used in organizations to indicate a person having a certain amount of authority, power, or importance in the organization. Secretary's announce important events and communicate to the organization. The term is derived from the Latin word secernere, "to distinguish" or "to set apart", the passive participle (secretum) meaning "having been set apart", with the eventual connotation of something private or confidential, as with the English word secret. A secretarius was a person, therefore, overseeing business confidentially, usually for a powerful individual (a king, pope, etc.).