  Let $p$ be a prime. We define $S(p)$ the smallest number $k$ such that every
positive integer is a sum of at most $k$ squares of integers that are not
divisible by $p$. In this article, we prove that $S(2)=10$, $S(3)=6$, $S(5)=5$,
and $S(p)=4$ for any prime $p$ greater than $5$. In particular, it is proved
that every positive integer is a sum of at most four squares not divisible by
$5$, except the unique positive integer $79$.
