The quantum Hall effect is one of the most fundamental quantum phenomena in condensed matter physics, and via tuning the quantum Hall states (QHSs), the evolution of band topologies and electron correlations can be investigated and revealed therein. However, for the vast majority of QHS systems, even- and odd-integer quantized plateaus coexist and cannot be effectively regulated. Here, we demonstrate field-effect-tunable even-odd transition of QHSs in a fixed two-unit-cell-thick (2-uc-thick) ${\mathrm{Bi}}_2{\mathrm{O}}_2\mathrm{Se}$ film, which is unlike irreversible thickness-controlled approaches [J. Wang et al., Even-integer quantum Hall effect in an oxide caused by a hidden Rashba effect, Nat. Nanotechnol. 19, 1452 (2024)]. Only even-integer quantized plateaus are observed in the 2-uc-thick epitaxial film on ${\mathrm{SrTiO}}_3$ when the quantum oscillations show degenerated spin splitting under positive gate voltages. In contrast, the simultaneous emergence of even- and odd-integer QHSs is achieved under negative gate voltages, accompanied with significant spin splitting. Theoretical calculations reveal that this reversible switching stems from gate-controlled inversion symmetry breaking that modulates the splitting of Landau levels. This Letter demonstrates the significant tunability of electrostatic gating in altering inversion symmetry and modulating electron correlations in Rashba-type 2-uc-thick ${\mathrm{Bi}}_2{\mathrm{O}}_2\mathrm{Se}$, enabling dynamic control of QHSs parity without structural changes, thereby extending the potential applications to fractional statistics and spintronics.

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• Electrical properties
• Landau levels
• Quantum phase transitions
• Quantum transport
• Rashba coupling
• Spin-orbit coupling
• Interfaces