Metal-organic frameworks (MOFs) possess an ordered and size-controllable porous structure, making them an interesting heterogeneous confining environment for water. Herein, molecular dynamics simulations are employed to investigate the structure of confined water vapor in zeolitic imidazolate framework-8 (ZIF-8) nanopores. Water dimers, which are rarely observed in liquid or water vapor, can form in ZIF-8 at room temperature. The six-ring-member gate is the main location of a water dimer in ZIF-8. The terminal methyl and CH groups of the imidazole linker interact with the water dimer by relatively weak hydrogen bonding. The above-presented findings provide a foundation for the elucidation of water confined in ZIF-8 and demonstrate the potential of obtaining low-order clusters of water by MOFs.