Provided are anti-PD-1 antibodies or fragments thereof. In various example, the antibodies or fragments thereof includes a heavy chain variable region comprising heavy chain complementarity determining regions HCDR1, HCDR2, and HCDR3, and a light chain variable region comprising light chain complementarity determining regions LCDR1, LCDR2, and LCDR3. Methods of using the antibodies or fragments thereof for treating and diagnosing diseases such as cancer, infection or immune disorders are also provided.