The CP properties of the Higgs boson are studied in the vector-boson fusion production mode. The analysis exploits the decay mode of the Higgs boson into two τ-leptons using 140 fb$^{−1}$ of proton-proton collision data at $ \sqrt{s}=13 $ TeV collected by the ATLAS experiment at the Large Hadron Collider. Results are obtained using the Optimal Observable method. CP-violating interactions between the Higgs boson and electroweak gauge bosons are considered in the effective field theory framework, with the interaction strength described in the HISZ basis by $ \overset{\sim }{d} $, and in the Warsaw basis by $ {c}_{H\overset{\sim }{W}} $, $ {c}_{H\overset{\sim }{B}} $, and $ {c}_{H\overset{\sim }{W}B} $. No deviations relative to the Standard Model are observed, and limits are obtained on the strength parameters. The $ \overset{\sim }{d} $ parameter is constrained to the interval [−0.012, 0.044] at the 95% confidence level while $ {c}_{H\overset{\sim }{W}} $ is constrained to [−0.24, 0.83], when considering both linear and quadratic effects of physics beyond the Standard Model.