Veteran Giggs is part of the United side that lead the league, two points ahead of City although Sir Alex Ferguson's team have two games in hand.
Arsenal are a further two points adrift and with this season's title run-in apparently wide open, Giggs knows City are serious contenders.
He told BBC Sport: "Of course they can [win it] with the quality of players they've got and the strength of depth they have.
"They are just able to go out buy players like Edin Dzeko in the January transfer window so you have to take them seriously. They are one of four or five teams that could win it."