Here we report a structural phase transition and its interplay with superconductivity in the suboxide ${\mathrm{La}}_5{\mathrm{Pb}}_3\mathrm{O}$. Upon cooling through $T_t=225\mathrm{K},{\mathrm{La}}_5{\mathrm{Pb}}_3\mathrm{O}$ transforms from a high-temperature $I4$/mcm to a low-temperature $P4$/ncc structure in which La-Pb dimerization along the $c$ axis occurs. This transition is accompanied by anomalies in the temperature dependence of electrical resistivity and specific heat. High-pressure electrical transport measurements reveal that hydrostatic pressure suppresses the structural transition and possibly induces superconductivity with a maximum superconducting temperature of 10 K. Density functional theory calculations show minimal changes in the electronic density of states and no gap opening at $E_F$ across $T_t$, suggesting that the transition is driven by bonding effects rather than Fermi surface instability. These findings establish ${\mathrm{La}}_5{\mathrm{Pb}}_3\mathrm{O}$ as a promising platform for exploring the interplay between weak structural transitions and superconductivity.

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• Electronic structure
• Superconducting phase transition
• Crystal structures
• Single crystal materials
• Crystal growth
• Density functional theory
• Neutron diffraction
• Pressure effects
• Resistivity measurements