states. In such states, the noise in one field quadrature is reduced below the vacuum level while the other quadrature becomes correspondingly more noisy, as required by Heisenberg’s uncertainty principle. Squeezed optical fields have been proposed and demonstrated to enhance the sensitivity of interferometric measurements beyond the photon shot-noise limit, with applications in gravitational wave detection. They have also been used to increase the sensitivity of atomic absorption spectroscopy, imaging, atom-based magnetometry, and particle tracking in biological systems. At microwave frequencies, cryogenic temperatures are required for the electromagnetic field to be in its vacuum state. Squeezed microwaves have been produced, used for fundamental studies of light-matter interaction and for enhanced sensing of a mechanical resonator, and proposed to enhance the sensitivity of the readout of superconducting qubits. Here we report the use of squeezed microwave fields to enhance the sensitivity of magnetic resonance spectroscopy of an ensemble of electronic spins. Our scheme consists in sending a squeezed vacuum state to the input of a cavity containing the spins while they are emitting an echo, with the phase of the squeezed quadrature aligned with the phase of the echo. We demonstrate a total noise reduction of 1.2\,dB at the spectrometer output due to the squeezing. These results provide a motivation to examine the application of the full arsenal of quantum metrology to magnetic resonance detection.