ThestructureandfunctionofBNS(bionanosystems) suchasmacromolecules, viruses and ribosomes are strongly affected by electrostatic interactions. Yet their supra-millionatomsizemakesthemdifﬁculttosimulateviaastraightforwardPoissonBoltzmann (PB) approach. Here we explore a multiscale approach that results in a coarse-grained PB equation that follows rigorously fromthe all-atomPB equation. The derivation of the coarse-grainedequation follows froman ansatz on the dependenceof the electrical potential in two distinct ways, i.e. one reﬂecting atomic-scale variations and the other capturing nanometer-scale features. With this ansatz and a series expansion of the potential in a length-scale ratio, the coarse-grained PB equation is obtained. This multiscale methodology and an efﬁcient computational methodology provide a way to efﬁciently simulate BNS electrostatics with atomic-scale resolution for the ﬁrst time, avoiding the need for excessive supercomputer resources. The coarse-grained PB equation contains a tensorial dielectric constant that mediates the channeling of the electric ﬁeld along macromolecules in an aqueous medium. The multiscale approach and novel salinity connections to the PB equation presented here should enhance the accuracy and wider applicability of PB modeling.