Magnetic sensing is a cornerstone of precision science, yet state-of-the-art sensors often encounter sensitivity constraints imposed by intrinsic noise and limited magnetic response. Here we introduce a magnetic amplification mechanism that boosts weak signals by several orders of magnitude using hyperpolarized spins. The principle is applicable to a broad range of hyperpolarized nuclear or electron spins, including those in noble gases, alkali metals, liquids, diamonds, and semiconductors. We propose a series of magnetic amplifiers and, as an initial experimental validation, demonstrate that noble-gas spins achieve magnetic amplification exceeding three orders of magnitude. We characterize the noise introduced during the amplification, showing that the amplified signal maintains exceptionally low noise levels. Consequently, we achieve enhanced magnetic sensitivity, reaching $4–6\mathrm{fT}/{\mathrm{Hz}}^{1/2}$ using a ${}^{129}\mathrm{Xe}$ amplifier and an unprecedented $0.8\mathrm{fT}/{\mathrm{Hz}}^{1/2}$ using a ${}^3\mathrm{He}$ amplifier. This sensitivity is on par with the most advanced magnetometers available today and holds the potential to be improved to $0.01\mathrm{fT}/{\mathrm{Hz}}^{1/2}$ under current experimental conditions. This work paves the way for ultrasensitive magnetic sensing and opens intriguing possibilities for various precision measurements.

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• Magnetometry
• Nuclear & electron resonance
• Quantum metrology
• Spin dynamics
• Atomic gases
• Magnetic moment
• Spin
• Optically detected magnetic resonance