  We present the explicit superfield realizations of the hidden $SU(4)$ and
$O(5)$ $R$-symmetries in $4D, {\cal N}=4$ and $5D, {\cal N}=2$ supersymmetric
Yang-Mills theories in the harmonic superspace approach. The $R$-symmetry
transformations are constructed and their algebraic structure is studied. It is
shown that such transformations are consistent with both manifest and hidden
supersymmetry transformations. These symmetries can serve as an alternative
tool for constructing the relevant complete low-energy superfield effective
actions determined earlier from the hidden supersymmetry considerations.
