Organic-inorganic lead halide perovskites have shown impressive power conversion efficiency (PCE) in a range of solar cell architectures. Despite the multiple ionic compositions that have been reported so far, the presence of organic constituents is an essential element in all the high efficiency formulations, with the methylammonium (MA) and formamidinium (FA) cations being the sole realistic options available to date. In this study, we demonstrate a novel three-dimensional (3D) perovskite with improved material stability as a result of the incorporation of an alternative organic cation, guanidinium, into the MAPb3 crystal structure. The new MA1_xGuaxPbl3 perovskite shows enhanced thermal stability and intrinsically new structural and optoelectronic properties. This allows for stable and high-power conversion efficiencies over 20%, a fundamental step within the perovskite field.