Reaction mechanisms for the hydrothermal synthesis of barium titanate are evaluated. Feedstocks of barium hydroxide octahydrate and anatase titania are reacted for varying durations (1–72 h) to provide intermediate‐stage samples for characterization by transmission electron microscopy/ energy‐dispersive spectroscopy (TEM/EDS), X‐ray diffractometry (XRD), and inductively coupled plasma spectroscopy (ICP). Quantitative evaluation of the extent of reaction by ICP and XRD methods permits the analysis of data with the Johnson‐Mehl‐Avrami equation. This analysis reveals two reaction‐rate regimes. Kinetic analysis, based on reaction progress, yields insight into the first reaction‐rate regime but is inconclusive in the analysis of the second reaction‐rate regime. In the first regime, at the early stage of barium titanate formation, a dissolution‐precipitation mechanism dominates. In contrast, in the second regime, at longer reaction times, an in‐situ transformation mechanism is probably dominant. However, multiple reaction mechanisms (e.g., in‐situ transformation and dissolution‐precipitation) may be competing for rate control. Alternatively, dissolution‐precipitation may be the dominant mechanism throughout the barium titanate synthesis, with nucleation and growth controlling the first regime and dissolution rate controlling the second regime.