The present disclosure provides isolated human monoclonal antibodies and antigen-binding portions thereof that bind to human LAG-3, in particular antibodies and antigen-binding antibody portions that can inhibit the binding of LAG-3 to MHC Class II molecules and that can stimulate antigen-specific T cell responses. Nucleic acid molecules encoding the antibodies and antigen-binding antibody portions of the invention as well as compositions and immunoconjugates of the antibodies and antigen-binding antibody portions of the invention are also provided. This disclosure also provides for the use of the antibodies and antigen-binding antibody portions of the invention in therapy.