A tool storage device includes a platform and a fastening seat. The platform has a first guide structure. The fastening seat has a first standing wall, a tongue projecting from the first standing wall, a second standing wall, and an interconnecting segment interconnecting the first and second standing walls. The fastening seat is rotatable relative to the platform between a first position where the first segment engages the first guide structure, and a second position where a distance between an end of the tongue which is opposite to the second standing wall and a center of gravity of the second standing wall varies from that when the fastening seat is at the first position.