We theoretically show that a magnet can be stably levitated on top of a punctured superconductor sheet in the Meissner state without applying any external field. The trapping potential created by such induced-only superconducting currents is characterized for magnetic spheres ranging from tens of nanometers to tens of millimeters. Such a diamagnetically levitated magnet is predicted to be extremely well isolated from the environment. We therefore propose to use it as an ultrasensitive force and inertial sensor. A magnetomechanical read-out of its displacement can be performed by using superconducting quantum interference devices. An analysis using current technology shows that force and acceleration sensitivities on the order of 10 −23 N/ √ Hz (for a 100 nm magnet) and 10 −14 g/ √ Hz (for a 10 mm magnet) might be within reach in a cryogenic environment. Such unprecedented sensitivities can be used for a variety of purposes, from designing ultra-sensitive inertial sensors for technological applications (e.g. gravimetry, avionics, and space industry), to scientific investigations on measuring Casimir forces of magnetic origin and gravitational physics.Most modern force and inertial sensors are based on the response of a mechanical oscillator to an external perturbation. Such sensors find applications in a wide range of domains: from measuring accelerations in smartphones and automobiles [1] in present-day technology, to being used on the cutting edge of research for magnetic resonance force microscopy [2][3][4], mass spectroscopy at the single-molecule level [5], and measuring gravitational and Casimir physics at short distances [6][7][8][9][10]. Most force and inertial sensors are based on microfabricated clamped mechanical oscillators, whose sensitivity is ultimately limited by mechanical dissipation due to material and clamping losses [11]. Levitation offers a clear route to avoiding these loss mechanisms. Indeed, the most precise commercial accelerometers are based on levitated systems: the superconducting gravimeter, which levitates a superconducting centimeter-sized sphere in the mixed superconducting state to achieve acceleration sensitivities of 3.1 × 10 −10 g/ √ Hz [12], and the MicroStar accelerometer, which electrostatically levitates a centimeter-sized cube in space leading to 10 −11 g/ √ Hz [13]. In research, different levitated systems are being explored to push into unexplored levels of sensitivity. This includes the demonstration of a record force sensitivity of 4 × 10 , and matter-wave interferometry using clouds of atoms with a sensitivity of ∼ 10 −9 g/ √ Hz [26,27].In this Letter, we aim at exploiting the exquisite isolation from the environment provided by magnetic levitation in a cryogenic environment. In particular, we propose an all-magnetic passively-levitated sensor that can be scaled over a broad range of sizes and is predicted to reach unprecedented ultra-high force and inertial sensitivities of 10 −23 N/ √ Hz and 10 −14 g/ √ Hz, respectively. We show that a spherical particle with a per...