A test of CP invariance in Higgs boson production via vector-boson fusion is performed in the H → ττ decay channel. This test uses the Optimal Observable method and is carried out using 36.1fb^-1 of √s=13 TeV proton–proton collision data collected by the ATLAS experiment at the LHC. Contributions from CP-violating interactions between the Higgs boson and electroweak gauge bosons are described by an effective field theory, in which the parameter d^˜  governs the strength of CP violation. No sign of CP violation is observed in the distributions of the Optimal Observable, and d^˜  is constrained to the interval [−0.090, 0.035] at the 68% confidence level (CL), compared to an expected interval of d^˜ ∈ [-0.035,0.033] based upon the Standard Model prediction. No constraints can be set d^˜ on at 95% CL, while an expected 95% CL interval of d^˜ ∈ [-0.21,0.15] for the Standard Model hypothesis was expected.