In the search for new cathode materials for sodium ion batteries, the exploration of polyanionic compounds has led to attractive candidates in terms of high redox potential and cycling stability. Herein we report the synthesis of the two new sodium transition-metal pentaborates Na3MB5O10 (M = Fe, Co), where Na3FeB5O10 represents the first sodium iron borate reported at present. By means of synchrotron X-ray diffraction, we reveal a layered structure consisting of pentaborate B5O10 groups connected through M2+ in tetrahedral coordination, providing possible three-dimensional Na-ion migration pathways. Inspired by these structural features, we examined the electrochemical performances versus sodium and showed that Na3FeB5O10 is active at an average potential of 2.5 V vs Na+/Na-0, correlated to the Fe3+/Fe2+ redox couple as deduced from ex situ Mossbauer measurements. This contrasts with the case for Na3CoB5O10, which is electrochemically inactive. Moreover, we show that their electrochemical performances are kinetically limited, as deduced by complementary ac/dc conductivity measurements, hence confirming once again the complexity in designing high-performance borate-based electrodes.