Antibodies and antigen-binding fragments thereof that immunospecifically bind to PD-1, such as human or mouse PD-1, and are agonists of PD-1 and can induce or promote an immune response that activates immune cell proliferation or activity. Unlike the majority of known anti-PD-1 and anti-PD-L1 antibodies, which reduce the suppression of PD-1 function by e.g., PD-L1 by physically blocking the interaction between PD-1 and an inactivating ligand, although not to be bound by theory, the disclosed antibodies and antigen-binding fragments thereof are currently best understood to immunospecifically bind to an epitope of PD-1 whereby PD-1 is activated directly. Methods for the treatment of cancer comprising administration of the antibodies or antigen-binding fragments to a subject in need thereof. Polynucleotides encoding the antibodies or antigen-binding fragments. Host cells comprising the polynucleotides.