We use a reservoir engineering technique based on two-tone driving to generate and stabilize a quantum squeezed state of a micron-scale mechanical oscillator in a microwave optomechanical system. Using an independent backaction-evading measurement to directly quantify the squeezing, we observe 4.7 AE 0.9 dB of squeezing below the zero-point level surpassing the 3 dB limit of standard parametric squeezing techniques. Our measurements also reveal evidence for an additional mechanical parametric effect. The interplay between this effect and the optomechanical interaction enhances the amount of squeezing obtained in the experiment. DOI: 10.1103/PhysRevLett.117.100801 Generating nonclassical states of a massive object has been a subject of considerable interest. It offers a route toward fundamental tests of quantum mechanics in an unexplored regime [1]. One of the most important and elementary quantum states of an oscillator is a squeezed state [2], which is a minimum uncertainty state that has a quadrature which is smaller than the zero-point level. Such states have long been discussed in the context of gravitational wave detection to improve the measurement sensitivity [3][4][5]. It is well known that a coherent parametric drive can be used to squeeze mechanical fluctuations [6,7], which is essentially equivalent to the technique first used to squeeze ground-state optical fields [8]. However, the maximum steady-state squeezing achieved by this method is limited to 3 dB due to the onset of parametric instability. Therefore, it is, in principle, impossible to have a steady state where the mechanical motion is squeezed below one half of the zero-point level using only parametric driving. These limitations may be overcome by combining continuous quantum measurement and feedback [9][10][11][12], but it would substantially increase the experimental complexity.Another method to generate a robust quantum state is quantum reservoir engineering [13], which has been used to generate quantum squeezed states and entanglement with trapped ions [14,15] and superconducting qubits [16]. It can also be applied to an optomechanical system to generate strong steady-state squeezing without quantumlimited measurement and feedback [17]. By modulating the optomechanical coupling with two imbalanced classical drive tones, the driven cavity acts effectively as a squeezed reservoir. When the engineered dissipation from the cavity dominates the dissipation from the environment, the mechanical resonator relaxes to a steady squeezed state. This technique has been applied recently to generate quantum squeezed states of macroscopic mechanical resonators [18][19][20].In addition to being a tool for state preparation, optomechanics also provides a means to probe the quantum behavior of macroscopic objects [21][22][23]. In particular, a backaction-evading (BAE) measurement [10,20,[24][25][26][27]] of a single motional quadrature can be implemented in an optomechanical system. If the drive tones that modulate the coupling are balanced, a continuou...