The invasive oomycete pathogen Aphanomyces astaci significantly threatens native European crayfish populations, prompting investigations towards the effects of protective immunostimulation on the immune response of the vulnerable noble crayfish (Astacus astacus). Here, we evaluate the effect of three oomycete-derived immunostimulant treatments: laminarin (β-1,3-glucan found within the Ap. astaci cell wall), inactivated Ap. astaci spores and Ap. astaci hyphal homogenate. Our findings reveal immediate changes in the noble crayfish total haemocyte count (THC), differential haemocyte count (DHC), and gene expression. A short-term increase in the THC was observed in all treatments, with a gradual return to normal values 8 h post immunostimulation. Granular haemocytes seem to be involved in response to immunostimulation with inactivated Ap. astaci spores, while the number of semi-granular and hyaline haemocytes increased in response to laminarin and Ap. astaci hyphal homogenate. Analysis of the differentially expressed genes showed that the Prophenoloxidase pathway genes and Toll pathway genes are involved in the response to oomycete-derived immunostimulants. Prolonged effects of immunostimulation were reflected in the decreased C/EBP and Kr-h1 gene expression in the hyphal homogenate group as well as decreased Kr-h1 expression in the spore group. Taken together, our results indicate that immunostimulation causes a dynamic change in the noble crayfish immune system response, with similarities in the gene expression patterns between immunostimulated and Ap. astaci infected noble crayfish. As a future research focus, we highlight the importance of molecular characterisation of the genes involved in the anti-oomycete response which could provide valuable insights into pathogen resistance in freshwater crayfish. In the context of the Ap. astaci mediated downfall of the noble crayfish stocks across Europe, further exploration is needed regarding the benefits of the oomycete-derived immunostimulation that can potentially support conservation and aquacultural efforts.