The present invention provides a treatment liquid that exhibits excellent washing properties and improves the smoothness of an object to be treated in a case where the treatment liquid is used for treating an object to be treated containing a cobalt-containing substance. The present invention also provides a method for treating an object to be treated. The treatment liquid according to an embodiment of the present invention contains water, hydroxylamine, and three kinds of first anions consisting of Cl−, NO2−, and NO3−, in which a total content of the first anions is 0.0001 to 30 parts by mass with respect to 100 parts by mass of the hydroxylamine.