The kaon nuclear optical potential is studied including the effect of the Θ^+ pentaquark. The one-nucleon contribution is obtained using an extension of the Jülich meson-exchange potential as bare kaon-nucleon interaction. Significant differences between a fully self-consistent calculation and the usually employed low-density Tρ approach are observed. The influence of the one-nucleon absorption process, K N →Θ^+, on the kaon optical potential is negligible due to the small width of the

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... k. In contrast, the two-nucleon mechanism, K N N →Θ^+ N, estimated from the coupling of the pentaquark to a two-meson cloud, provides the required amount of additional kaon absorption to reconcile with data the systematically low K^+-nucleus reaction cross sections found by the theoretical models.