We report nonreciprocal charge transport in epitaxial monoclinic ${\mathrm{SrIrO}}_3$ ($m$-SIO) thin films, a nonsymmorphic symmetry-protected Dirac semimetal with strong spin-orbit coupling. Second-harmonic magnetotransport measurements reveal both bilinear magnetoelectric resistance and a nonlinear planar Hall effect, providing clear evidence of spin-momentum locked states in $m$-SIO. Systematic angular-dependent measurements suggest a predominantly in-plane spin vector oriented perpendicular to the electron momentum, consistent with Rashba-type spin-momentum locking on the Fermi surface. Quantitative analysis of the bilinear magnetoelectric resistance based on a simplified Rashba model yields a large effective Rashba parameter, suggesting that the Rashba-type surface states are closely related to the strongly spin-orbit coupled electronic structure of the nonsymmorphic Dirac bulk states. Our results provide a transport route to probe Rashba-type spin-momentum locking in $m$-SIO and identify $m$-SIO as a promising platform for oxide spin-orbitronics.

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• Magnetotransport
• Rashba coupling
• Spin-orbit coupling
• Iridates
• Thin films