Provided are anti-PD-1 antibodies or fragments thereof. In various example, the antibodies or fragments thereof include heavy chain variable regions comprising heavy chain complementarity determining regions HCDR1 (SEQ ID NO: 1, 7 or 13), HCDR2 (SEQ ID NO: 2, 8 or 14), and HCDR3 (SEQ ID NO: 3, 9 or 15), and light chain variable regions comprising light chain complementarity determining regions LCDR1 (SEQ ID NO: 4, 10 or 16), LCDR2 (SEQ ID NO: 5, 11 or 17), and LCDR3 (SEQ ID NO: 6, 12 or 18). Methods of using the antibodies or fragments thereof for treating and diagnosing diseases such as cancer, infection, or immune disorders are also provided.