A biodegradable polyester resin, which comprises a first repeat unit comprising a first diol residue and an aromatic dicarboxylic acid residue and a second repeat unit comprising a second diol residue and an aliphatic dicarboxylic acid residue, wherein the first and second diol residues each comprise a residue of 1,4-butanediol, 1,2-ethanediol, 1,3-propanediol, or a derivative thereof, the aromatic dicarboxylic acid residue comprises a residue of terephthalic acid, dimethyl terephthalate, or a derivative thereof, the aliphatic dicarboxylic acid residue comprises a residue of adipic acid, succinic acid, sebacic acid, or a derivative thereof, the ratio (X/Y) of the number of the first repeat unit to the number of the second repeat unit is 0.6 to 1.8, the number of the first repeat unit is 100 to 900, the number of the second repeat unit is 100 to 1,100, and the softness index represented by the following Equation 1 is 90 to 130: Softness index SI = Tc ∘ C / 100 + VST C ∘ in Equation 1, Tc and VST are values, exclusive of units, measured for a specimen of a biodegradable polyester film prepared from the biodegradable polyester resin, Tc is the crystallization temperature (°C) measured using a differential scanning calorimeter while the temperature is elevated from 40°C to 180°C at a rate of 10°C/minute and then lowered to -50°C at a rate of 10°C/minute, and VST is the temperature (°C) when the tip of a needle passes through 1 mm of a specimen having a thickness of 5 mm and a width of 10 mm while the specimen is measured at 50°C/hour and a load of 10 N according to ASTM D1525; and wherein the biodegradable polyester resin has a loss tangent (tan δ) of greater than 1 represented by the following Equation 3: Loss tangent tan δ = G " / G ′ in Equation 3, G' is the storage modulus of a biodegradable polyester sheet prepared from the biodegradable polyester resin at 240°C and a frequency of 5 rad/s in dynamic viscoelasticity measurement, and G" is the loss modulus of the biodegradable polyester sheet prepared from the biodegradable polyester resin at 240°C and a frequency of 5 rad/s in dynamic viscoelasticity measurement. wherein the first and second diol residues each comprise a residue of 1,4-butanediol, 1,2-ethanediol, 1,3-propanediol, or a derivative thereof, the aromatic dicarboxylic acid residue comprises a residue of terephthalic acid, dimethyl terephthalate, or a derivative thereof, the aliphatic dicarboxylic acid residue comprises a residue of adipic acid, succinic acid, sebacic acid, or a derivative thereof, the ratio (X/Y) of the number of the first repeat unit to the number of the second repeat unit is 0.6 to 1.8, the number of the first repeat unit is 100 to 900, the number of the second repeat unit is 100 to 1,100, and the softness index represented by the following Equation 1 is 90 to 130: Softness index SI = Tc ∘ C / 100 + VST C ∘ in Equation 1, Tc and VST are values, exclusive of units, measured for a specimen of a biodegradable polyester film prepared from the biodegradable polyester resin, Tc is the crystallization temperature (°C) measured using a differential scanning calorimeter while the temperature is elevated from 40°C to 180°C at a rate of 10°C/minute and then lowered to -50°C at a rate of 10°C/minute, and VST is the temperature (°C) when the tip of a needle passes through 1 mm of a specimen having a thickness of 5 mm and a width of 10 mm while the specimen is measured at 50°C/hour and a load of 10 N according to ASTM D1525; and wherein the biodegradable polyester resin has a loss tangent (tan δ) of greater than 1 represented by the following Equation 3: Loss tangent tan δ = G " / G ′ in Equation 3, G' is the storage modulus of a biodegradable polyester sheet prepared from the biodegradable polyester resin at 240°C and a frequency of 5 rad/s in dynamic viscoelasticity measurement, and G" is the loss modulus of the biodegradable polyester sheet prepared from the biodegradable polyester resin at 240°C and a frequency of 5 rad/s in dynamic viscoelasticity measurement. Tc is the crystallization temperature (°C) measured using a differential scanning calorimeter while the temperature is elevated from 40°C to 180°C at a rate of 10°C/minute and then lowered to -50°C at a rate of 10°C/minute, and VST is the temperature (°C) when the tip of a needle passes through 1 mm of a specimen having a thickness of 5 mm and a width of 10 mm while the specimen is measured at 50°C/hour and a load of 10 N according to ASTM D1525; and wherein the biodegradable polyester resin has a loss tangent (tan δ) of greater than 1 represented by the following Equation 3: Loss tangent tan δ = G " / G ′ in Equation 3, G' is the storage modulus of a biodegradable polyester sheet prepared from the biodegradable polyester resin at 240°C and a frequency of 5 rad/s in dynamic viscoelasticity measurement, and G" is the loss modulus of the biodegradable polyester sheet prepared from the biodegradable polyester resin at 240°C and a frequency of 5 rad/s in dynamic viscoelasticity measurement.