A test of CP invariance in Higgs boson production via vector-boson fusion in the $H\rightarrow\tau\tau$ decay mode is presented. This test uses the Optimal Observable method and is carried out using 36.1\,\mathrm{fb}^{-1} of \sqrt{s}=13\,\mathrm{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 $\tilde{d}$ governs the strength of CP violation. No sign of CP violation is observed in the distributions of the Optimal Observable, and $\tilde{d}$ is constrained to the interval [$-0.090$, $0.035$] at the 68% confidence level (CL), compared to an expected interval of $\tilde{d} \in [-0.035,0.033]$ based upon the Standard Model prediction. No constraints can be set on $\tilde{d}$ at 95% CL, while an expected 95% CL interval of $\tilde{d} \in [-0.21,0.15]$ for the Standard Model hypothesis was expected.

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