Stable silica-based extra-large-pore (ELP) zeolites are highly desirable for processing bulky molecules but remain exceedingly challenging to synthesize directly. Here, we report JU-87, a stable, fully connected pure-silica ELP zeolite formed through a 3D-to-3D topotactic condensation of an interrupted 24-membered-ring zeolite precursor (JU-86). The calcination-induced full condensation of the SiO^–···HOSi defects in JU-86 yields JU-87, which features one-dimensional 18-membered-ring channels (max. dimension: 17.36 Å) lined with pocket-like cavities and contains abundant, unprecedented, highly strained three-membered-ring units. Owing to its spacious channels, JU-87 can accommodate bulky dye molecules, demonstrating its potential for the adsorption and catalytic upgrading of large substrates.