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Abstract A mechanism of nucleoside triphosphorylation would have been critical in an evolving “RNA world” to provide high‐energy substrates for reactions such as RNA polymerization. However, synthetic approaches to produce ribonucleoside triphosphates (rNTPs) have suffered from conditions such as high temperatures or high pH that lead to increased RNA degradation, as well as substrate production that cannot sustain replication. Previous reports have demonstrated that cyclic trimetaphosphate (cTmp) can react with nucleosides to form rNTPs under prebiotically‐relevant conditions, but their reaction rates were unknown and the influence of reaction conditions not well‐characterized. Here we established a sensitive assay that allowed for the determination of second‐order rate constants for all four rNTPs, ranging from 1.7×10⁻⁶to 6.5×10⁻⁶ M⁻¹ s⁻¹. The ATP reaction shows a linear dependence on pH and Mg²⁺, and an enthalpy of activation of 88±4 kJ/mol. At millimolar nucleoside and cTmp concentrations, the rNTP production rate is sufficient to facilitate RNA synthesis by both T7 RNA polymerase and a polymerase ribozyme. We suggest that the optimized reaction of cTmp with nucleosides may provide a viable connection between prebiotic nucleotide synthesis and RNA replication.