A hardware True Random Number Generator (TRNG) yields random numbers from a physical process. Traditionally, such devices are based on statistically random signals such as thermal noise or other quantum phenomena. In this paper we propose an innovative TRNG design using a Spin Transfer Torque Magnetic Tunnel Junction (STT-MTJ) device. We exploit the stochastic nature of the MTJ device switching, and perform on-the-fly temperature/current variation compensation. We show that the proposed solution keeps up with environmental changes and generates random sequences with high probability.