Real Madrid are desperate to land Ronaldo this summer but Giggs insisted to The Sun: "He is at the best club.
"Cristiano has proved he is the best player in the world over the last couple of years and he has done that at United, so why leave? He's still a young player, still learning the game and can still improve. He can do that at United."
Ronaldo sees the challenge of reviving Madrid's fortunes as too good to turn down.
But Giggs said: "Real Madrid will come back purely because of the club they are and the history they have got.
"But United are more stable in the fact there are no presidential elections and the manager's been here so long."