There is a growing interest in designing electrocatalysts for highly selective electroreduction of CO₂ to deeply reduced products such as CH₄. However, CO₂-to-CH₄ conversion remains a great challenge in controlling the multi-electron/proton transfer and the adsorption of key intermediates. In this work, a metallacycle-based porous crystal (1) was synthesized for selective CO₂-to-CH₄ electroconversion in aqueous solution. The single-crystal analysis results disclosed that the metallacycle module of 1 features open metal sites and could be self-adaptive to encapsulate guest molecules of different sizes in its cavity. These enable the metallacycle to function as a nano-reactor to mimic the enzymatic cavity that is surrounded by catalytically active sites and adaptively bind to diverse substrates and intermediates. Accordingly, 1 exhibits desirable CO₂ capture capacity, and its porous framework further promotes CO₂ diffusion to catalytic sites. The above merits bring about an excellent CH₄ selectivity of 70% with a partial current density of 10.3 mA cm⁻² in a H-type cell at −1.5 V (versus the reversible hydrogen electrode), while a CH₄ selectivity up to 81% with a partial current density of 23 mA cm⁻² in a flow cell. Mechanistic investigations suggested that the adaptive cavity and the synergistic effect of aromatic hydrogen atoms, via hydrogen-bonding interactions, are beneficial to stabilize the key intermediates of CO₂-to-CH₄ conversion. This work develops a novel platform to regulate the reactivity and selectivity of CO₂ electroreduction reactions.