The $p$-norm of $r$-matrices generalizes the $2$-norm of $2$-matrices. It is shown that if a nonnegative $r$-matrix is symmetric with respect to two indices $j$ and $k$, then the $p$-norm is attained for some set of vectors such that the $i$th and the $j$th vectors are identical. It follows that the $p$-spectral radius of a symmetric nonnegative $r$-matrix is equal to its $p$-norm for any $p\geq2$.