An electrical seatpost assembly comprises a first cylinder, a second cylinder, a positioning structure, an electrical actuator, and a power supply. The second cylinder is telescopically received in the first cylinder. The positioning structure is to adjustably position the second cylinder relative to the first cylinder in a telescopic direction. The electrical actuator is to actuate the positioning structure in accordance with an electrical signal. The power supply is to supply electrical power to the electrical actuator. The power supply and the electrical actuator are disposed in one of the first cylinder and the second cylinder to at least partly overlap with each other when viewed from a direction perpendicular to the telescopic direction.