Sensitive measurement of electrical signals is at the heart of modern science and technology. According to quantum mechanics, any detector or amplifier is required to add a certain amount of noise to the signal, equaling at best the energy of quantum fluctuations [1, 2]. The quantum limit of added noise has nearly been reached with superconducting devices which take advantage of nonlinearities in Josephson junctions [3, 4]. Here, we introduce a new paradigm of amplification of microwave signals with the help of a mechanical oscillator. By relying on the radiation pressure force on a nanomechanical resonator [5][6][7], we provide an experimental demonstration and an analytical description of how the injection of microwaves induces coherent stimulated emission and signal amplification. This scheme, based on two linear oscillators, has the advantage of being conceptually and practically simpler than the Josephson junction devices, and, at the same time, has a high potential to reach quantum limited operation. With a measured signal amplification of 25 decibels and the addition of 20 quanta of noise, we anticipate near Since the early days of quantum mechanics, the effect of quantum zero point fluctuations on measurement accuracy has been actively investigated. When measuring a position x of an object, one necessarily disturbs its subsequent motion by introducing a disturbance to the momentum p. The imprecision and disturbance are related by the fundamental limit ∆x∆p ≥ /2 owing to the Heisenberg uncertainty principle. A proper compromise between the two leads to the lowest added noise power per unit bandwidth ω/2 which equals the quantum fluctuations of the system itself at the signal frequency ω. On the other hand, if only one observable is measured, for example position, or a single quadrature such as either amplitude or phase of oscillations, noise in this measurement can be squeezed below the quantum limit at the expense of increased noise in the other quadrature. In this case, the amplifier is said to be phase-sensitive.While most modern transistors operate several orders of magnitude above the fundamental noise limit, superconducting Josephson junction parametric amplifiers [3, 4,8,9], working near the absolute zero of temperature, have found uses at the level of only a few added quanta at microwave frequencies.Approaching the quantum limit with a mechanical amplifier has remained fully elusive, moreover, there is little work whatsoever on amplifying electrical signals by mechanical means [10], foremost due to the typically small electromechanical interaction. In this work, we describe a way to approach quantum-limited microwave amplification, now with a mechanical device. Our system consists of a mechanical resonator affected by radiation pressure forces due to an electromagnetic field confined in a lithographically patterned thin-film microwave cavity. Depending on the configuration, it is capable of either phase-sensitive, or phase-insensitive amplification.Our system of two coupled linear oscillators form...