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Pressure, a fundamental thermodynamic variable, enables phase transitions to exotic phases with unique physical properties, such as superconductivity. In this work we perform a complete study of crystal structures and relevant electronic properties of La–S crystalline systems in a pressure range of 0–200 GPa. A structural search based on first-principles swarm-intelligence identifies two hitherto unknown pressure-stabilized stoichiometries, namely, LaS3 and LaS5, in addition to the previously reported compounds. We find that the S-S bonding patterns in La–S compounds evolve in the following sequence with increasing S content and pressure: Atomic S, S2 dimers, one-dimensional linear S chains, and two-dimensional S ladders. Further electron-phonon calculations show that both LaS3 and LaS5 are superconductors with critical temperatures of 13.6 K at 100 GPa and 11 K at 120 GPa, respectively. The softened acoustic phonon branches are responsible for their superconductivity. Our current work is expected to guide future experimental studies investigating superconductivity and structural features of La–S system and more, in general, of other rare-earth chalcogenides.