•A rope-spun rotor is proposed to convert low-frequency vibrations to rotation motion.•High-speed rotation is achieved by the rotor under ultralow-frequency vibrations.•A harvester with the rotor generates high power under ultralow-frequency excitations.•8-minute running of the harvester sustains 60-minute operation of a wireless sensor.•Three electronics are simultaneously operated by the harvester as tapped by hand. Ultralow-frequency kinetic energy mainly in the form of vibrations is omnipresent in the environment, but its effective exploitation remains a challenge. To tackle this problem, this paper presents a rope-spun rotor structure to transform ultralow-frequency vibrations/linear motions to rapid rotations through a piece of rope. The superior performance of the rotor is demonstrated by applying it to electromagnetic energy harvesting from ultralow-frequency vibrations and irregular human body motions. When the rope-spun rotor based harvester is periodically pushed down 10.5 mm at 1.5 Hz, it produces 9.7 mW electric power. When embedded in a shoe insole, the harvester delivers 8 mW power to a matched load as a male participant walks with the shoe at 6.5 km/h. When positioned under a piece of floorboard, the harvester can charge a supercapacitor (220 mF) from 0 to 3.5 V within 8 min. The harvester can also sustain the continuous operation of multiple electronics simultaneously by scavenging energy from gentle finger tapping motions. This study demonstrates a new mechanism for realizing vibration-to-rotation conversion and a promising way for efficient harvesting of ultralow-frequency energy.