An absorbent article for a pet comprising: a liquid permeable top surface layer, a liquid impermeable back surface layer, and an absorbent core disposed between the top surface layer and the back surface layer; the absorbent article for a pet being configured in a rectangular shape having a first end portion and a second end portion opposite each other, and a pair of side portions opposite each other orthogonal to the pair of the end portions (11, 12), the absorbent article for pet being configured to be worn in a state of being wrapped around the waist of the pet; the absorbent article further comprising: a pair of side sheets (51, 52) disposed on respective top surface layer sides of the pair of side portions, with an outer edge joined with the top surface layer or the back surface layer and with at least a part of an inner edge being a free end; an elastic member attached to a vicinity of an inner edge of the pair of side sheets (51, 52); and a pair of pocket portions (14, 15) formed between an inner face of the pair of side sheets (51, 52) and an outer face of the top surface layer, wherein the absorbent core comprises: a belt-like absorbent core main body that extends from a first end portion side to a second end portion side; and a first extension portion that extends outwards in a width direction from a first side edge in a central portion in a longitudinal direction of the absorbent core main body; a length of the pocket portions (14, 15) in the longitudinal direction of the absorbent article for pet is greater than a length of the first extension portion in the longitudinal direction of the absorbent article for a pet; an inner edge of a first side sheet, among the pair of side sheets (51, 52), disposed on a side where the first extension portion is provided, is not joined to the top surface layer and is a free end spanning an overall length in the longitudinal direction; an inner edge of a second side sheet, among the pair of side sheets, is joined to the top surface layer in the first end portion and the second end portion; and the inner edge of the second side sheet is a free end, except at the first end portion and the second end portion. a pair of side sheets (51, 52) disposed on respective top surface layer sides of the pair of side portions, with an outer edge joined with the top surface layer or the back surface layer and with at least a part of an inner edge being a free end; an elastic member attached to a vicinity of an inner edge of the pair of side sheets (51, 52); and a pair of pocket portions (14, 15) formed between an inner face of the pair of side sheets (51, 52) and an outer face of the top surface layer, wherein the absorbent core comprises: a belt-like absorbent core main body that extends from a first end portion side to a second end portion side; and a first extension portion that extends outwards in a width direction from a first side edge in a central portion in a longitudinal direction of the absorbent core main body; a length of the pocket portions (14, 15) in the longitudinal direction of the absorbent article for pet is greater than a length of the first extension portion in the longitudinal direction of the absorbent article for a pet; an inner edge of a first side sheet, among the pair of side sheets (51, 52), disposed on a side where the first extension portion is provided, is not joined to the top surface layer and is a free end spanning an overall length in the longitudinal direction; an inner edge of a second side sheet, among the pair of side sheets, is joined to the top surface layer in the first end portion and the second end portion; and the inner edge of the second side sheet is a free end, except at the first end portion and the second end portion.