Let
be a bounded domain with sufficiently smooth boundary
and
be a symmetric space defined on the measure space
. We consider a Dirichlet problem for
‐th order polyharmonic equation, and we establish its solvability (in strong sense) in Sobolev space
generated by the norm of
. Such spaces include classical Sobolev spaces, Orlicz–Sobolev spaces, grand Sobolev spaces, and Marcinkiewicz–Sobolev spaces. The obtained results are new for these special cases, too.