The authors experimentally observe many-body localization (MBL) in a one-dimensional quasi-random optical lattice of interacting fermions. They identify the MBL transition by monitoring the relaxation dynamics of an initially prepared charge density wave (CDW). For weak disorder, the system thermalizes, erasing the initial order. However, above a critical disorder strength, a significant portion of the initial ordering persists, serving as an effective order parameter for localization. The stationary density wave order and the critical disorder value depend on the interaction strength, in agreement with numerical simulations. This dependence is linked to the logarithmic growth of entanglement entropy, characteristic of the MBL phase. The study provides experimental evidence for ergodicity breaking due to MBL and opens avenues for further investigations, such as using true random disorder and extending to higher dimensions.The authors experimentally observe many-body localization (MBL) in a one-dimensional quasi-random optical lattice of interacting fermions. They identify the MBL transition by monitoring the relaxation dynamics of an initially prepared charge density wave (CDW). For weak disorder, the system thermalizes, erasing the initial order. However, above a critical disorder strength, a significant portion of the initial ordering persists, serving as an effective order parameter for localization. The stationary density wave order and the critical disorder value depend on the interaction strength, in agreement with numerical simulations. This dependence is linked to the logarithmic growth of entanglement entropy, characteristic of the MBL phase. The study provides experimental evidence for ergodicity breaking due to MBL and opens avenues for further investigations, such as using true random disorder and extending to higher dimensions.