  We calculate the Spencer cohomology of the $(1,0)$ Poincar\'e superalgebras
in six dimensions: with and without R-symmetry. As the cases of four and eleven
dimensions taught us, we may read off from this calculation a Killing spinor
equation which allows the determination of which geometries admit rigidly
supersymmetric theories in this dimension. We prove that the resulting Killing
spinors generate a Lie superalgebra and determine the geometries admitting the
maximal number of such Killing spinors. They are divided in two branches. One
branch consists of the lorentzian Lie groups with bi-invariant metrics and, as
a special case, it includes the lorentzian Lie groups with a self-dual Cartan
three-form which define the maximally supersymmetric backgrounds of $(1,0)$
Poincar\'e supergravity in six dimensions. The notion of Killing spinor on the
other branch does not depend on the choice of a three-form but rather on a
one-form valued in the R-symmetry algebra. In this case, we obtain three
different (up to local isometry) maximally supersymmetric backgrounds, which
are distinguished by the causal type of the one-form.
