A device for heating the water of an above-ground pool, such as a spa or an above-ground swimming pool, said heating device comprising: - a frame which has: - a base which is intended to rest on the ground when the heating device is in use, - a rear face, which rises from the base and which is adapted to rest against the pool when in use; - a front face, which rises from the base, being opposite to the rear face, and - an upper face, which connects the rear and front faces and which generally has a stepped shape, having at least two levels, which, in use, are at different respective heights and the highest level of which opens onto the rear face while the other level or levels (N1, N2) are distributed substantially uniformly between the base and the highest level, so that, in use, a user can access the pool by walking successively on the levels of the stepped shape by ascending these levels (N1, N2, N3) from the ground to enter the pool and descending these levels to the ground to exit the pool, - an air-water heat pump, which is adapted to transfer heat from the air to the water of the pool, the heat pump being arranged in an internal volume of the frame, which is defined between the base and the rear, front and upper faces, and through which air from outside the frame circulates, and - connection means for connecting to the pool, adapted both to supply water of the pool from outside the frame to the heat pump and to return the water heated by the heat pump to the outside of the frame to supply the pool. - a frame which has: - a base which is intended to rest on the ground when the heating device is in use, - a rear face, which rises from the base and which is adapted to rest against the pool when in use; - a front face, which rises from the base, being opposite to the rear face, and - an upper face, which connects the rear and front faces and which generally has a stepped shape, having at least two levels, which, in use, are at different respective heights and the highest level of which opens onto the rear face while the other level or levels (N1, N2) are distributed substantially uniformly between the base and the highest level, so that, in use, a user can access the pool by walking successively on the levels of the stepped shape by ascending these levels (N1, N2, N3) from the ground to enter the pool and descending these levels to the ground to exit the pool, - a base which is intended to rest on the ground when the heating device is in use, - a rear face, which rises from the base and which is adapted to rest against the pool when in use; - a front face, which rises from the base, being opposite to the rear face, and - an upper face, which connects the rear and front faces and which generally has a stepped shape, having at least two levels, which, in use, are at different respective heights and the highest level of which opens onto the rear face while the other level or levels (N1, N2) are distributed substantially uniformly between the base and the highest level, so that, in use, a user can access the pool by walking successively on the levels of the stepped shape by ascending these levels (N1, N2, N3) from the ground to enter the pool and descending these levels to the ground to exit the pool, - an air-water heat pump, which is adapted to transfer heat from the air to the water of the pool, the heat pump being arranged in an internal volume of the frame, which is defined between the base and the rear, front and upper faces, and through which air from outside the frame circulates, and - connection means for connecting to the pool, adapted both to supply water of the pool from outside the frame to the heat pump and to return the water heated by the heat pump to the outside of the frame to supply the pool.