Thermostable DNA ligases with enhanced DNA binding activity and reaction activity are obtained. These modified thermostable DNA ligases having enhanced DNA binding activity compared to the wild type can be obtained by substituting a negatively charged amino acid (for example, the amino acid corresponding to the aspartic acid at position 540 of SEQ ID NO: 1) present at the N-terminal side of the C-terminal helix moiety of thermostable DNA ligases from thermophilic bacteria, hyperthermophilic bacteria, thermophilic archaea, or hyperthermophilic archaea, with a non-negatively charged amino acid (for example, alanine, serine, arginine, or lysine).