Abstract SiO2 is a fundamental component of planetary interiors, yet its high‐pressure melting and phase relations remain uncertain. We develop a machine learning potential with first‐principles accuracy and perform large‐scale two‐phase coexistence simulations to determine the melting curves of stishovite, post‐stishovite, and seifertite up to 160 GPa with ∼10 K uncertainty. This unprecedented precision allows us to locate a tricritical point at 110 GPa/6060 K (stishovite/post‐stishovite/liquid) and a triple point at 130 GPa/6220 K (post‐stishovite/seifertite/liquid), which anchor the phase boundaries of silica’s high‐pressure polymorphs. In subducting slabs, the stishovite to post‐stishovite transition occurs at ∼1,510–1,640 km depth (∼65 GPa and 1930 K), consistent with previous estimates, whereas the post‐stishovite to seifertite transition occurs much shallower than previously inferred, at ∼600 km above the core‐mantle boundary (∼101 GPa and 2410 K). These results imply that seifertite is the thermodynamically stable silica polymorph throughout most of the deep lower mantle.