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 crystal, which shows metallic out-of-plane charge transport with high electrical conductivity. The material comprises columnar π arrays with an interlayer distance of 3.59 Å and periodic rhombohedral lattices formed by interwoven polyaniline chains. Electron spin resonance spectroscopy reveals significant electron delocalization in the 2DPANI lattices. First-principles calculations indicate the in-plane 2D conjugation and strong interlayer electronic coupling in 2DPANI facilitated by the Cl-bridged layer stacking. To assess the local optical conductivity, we used terahertz and infrared nanospectroscopy to unravel a Drude-type conductivity with an infrared plasma frequency and an extrapolated local d.c. conductivity of around 200 S cm−1. Conductive scanning probe microscopy showed an unusually high out-of-plane conductivity of roughly 15 S cm−1. Transport measurements through vertical and lateral micro-devices revealed comparable high out-of-plane (roughly 7 S cm−1) and in-plane conductivity (roughly 16 S cm−1). The vertical micro-devices further showed increasing conductivity with decreasing temperature, demonstrating unique out-of-plane metallic transport behaviour. By using this multilayer-stacked 2D conducting polymer design, we predict the achievement of strong electronic coupling beyond in-plane interactions, potentially reaching three-dimensional metallic conductivity6,7. A multilayer-stacked two-dimensional polyaniline crystal shows high electrical conductivity and unique out-of-plane metallic transport behaviour, indicating potential for strong electronic coupling beyond in-plane interactions and three-dimensional metallic conductivity.){kind=link}