The aim of the article is to show a Hörmander spectral multiplier theorem for an operator $A$ whose kernel of the semigroup $\exp(-zA)$ satisfies certain Poisson estimates for complex times $z.$ Here $\exp(-zA)$ acts on $L^p(\Omega),\,1 < p < \infty,$ where $\Omega$ is a space of homogeneous type with the additional condition that the measure of annuli is controlled. In most of the known Hörmander type theorems in the literature, Gaussian bounds and self-adjointness for the semigroup are needed, whereas here the new feature is that the assumptions are the to some extend weaker Poisson bounds, and $\HI$ calculus in place of self-adjointness. The order of derivation in our Hörmander multiplier result is typically $\frac{d}{2},$ $d$ being the dimension of the space $\Omega.$ Moreover the functional calculus resulting from our Hörmander theorem is shown to be $R$-bounded. Finally, the result is applied to some examples.