Provided are: a sludge dehydrating agent which can exert a steady dehydrating effect on various types of sludge and has excellent dehydration performance; and a sludge dehydration method using the sludge dehydrating agent. The sludge dehydrating agent of the present invention with a polymer which has a constituent unit derived from a cationic monomer, wherein a value k determined from an intrinsic viscosity [η] 1 [dL/g] of a 1-mol/L aqueous sodium nitrate solution of the polymer at 30° C. and an intrinsic viscosity [η] 0.01 [dL/g] of a 0.01-mol/L aqueous sodium nitrate solution of the polymer at 30° C. in accordance with formula (I) is 0.8 to 2.2 exclusive

k=([η]0.01−[η]1)/9  (I).