Physically Unclonable Functions (PUFs) are emerging cryptographic primitives used to implement low-cost device authentication and secure secret key generation. Weak PUFs (i.e., devices able to generate a single signature or to deal with a limited number of challenges) are widely discussed in literature. One of the most investigated solutions today is based on SRAMs. However, the rapid development of low power, high density, high performance SoCs has pushed the embedded memories to their limits and opened the field to the development of emerging memory technologies. The Spin-Transfer-Torque Magnetic Random Access Memory (STT-MRAM) has emerged as a promising choice for embedded memories due to its reduced read/write latency and high CMOS integration capability. In this paper, we propose an innovative PUF design based on STT-MRAM memory. We exploit the high variability affecting the electrical resistance of the Magnetic Tunnel Junction (MTJ) device in anti-parallel magnetization. We will demonstrate that the proposed solution is robust, unclonable and unpredictable.