Novel lignin solvents were developed by adding H2O to diethylene glycol (DEG). The solubility of lignin in the DEG/H2O solvents was determined at 25 °C, and the effect of mass ratio of H2O to DEG on lignin solubility was investigated. The DEG/H2O solvents exhibited highly efficient capacity for lignin dissolution, even at room temperature. The possible dissolution mechanism is proposed to be the interaction between the DEG and lignin. In addition, the DEG/H2O solvents hardly disrupt the structure of lignin.