We investigate the fine-structure [C
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Abstract ii ] line at 158μ m as a molecular gas tracer by analyzing the relationship between molecular gas mass (M mol) and [Cii ] line luminosity (L [CII ]) in 11,125z ≃ 6 star-forming, main-sequence galaxies from thesimba simulations, with line emission modeled by the Simulator of Galaxy Millimeter/Submillimeter Emission. Though most (∼50%–100%) of the gas mass in our simulations is ionized, the bulk (>50%) of the [Cii ] emission comes from the molecular phase. We find a sublinear (slope 0.78 ± 0.01) relation, in contrast with the linear relation derived from observational samples of more massive, metal-rich galaxies atz ≲ 6. We derive a median [Cii ]-to-M molconversion factor ofα [CII ]≃ 18M ⊙/L ⊙. This is lower than the average value of ≃30M ⊙/L ⊙derived from observations, which we attribute to lower gas-phase metallicities in our simulations. Thus, a lower, luminosity-dependent conversion factor must be applied when inferring molecular gas masses from [Cii ] observations of low-mass galaxies. For our simulations, [Cii ] is a better tracer of the molecular gas than COJ = 1–0, especially at the lowest metallicities, where much of the gas isCO-dark . We find thatL [CII ]is more tightly correlated withM molthan with star formation rate (SFR), and both the and relations arise from the Kennicutt–Schmidt relation. Our findings suggest thatL [CII ]is a promising tracer of the molecular gas at the earliest cosmic epochs.