Manchester City are attempting to get over the disappointment of missing out on Kaka - with the prospect of more signings to come.
De Jong, a former team-mate of current Blues star Vincent Kompany, is expected to bring added steel to the City midfield which has lacked a physical presence.
The Dutchman said of his move: "I feel very good and for me it is the next step in my own career after three years in Germany and I hope to be successful at Manchester City.
"The ideas and the planning behind this club are very hopeful and it is a new adventure so I am delighted to be here right now."
The former Ajax star has become Hughes' third signing of the transfer window - following Wayne Bridge and Craig Bellamy, whose arrival came just hours before Kaka's snub on Monday night.
Now executive chairman Garry Cook will turn his attention to manager Mark Hughes' other targets.
Newcastle goalkeeper Shay Given is high on the list - while interest has been registered in Chelsea's out-of-favour defender Branislav Ivanovic, who is also attracting the attention of Inter Milan.
"We had very clear plans going into this transfer window and we are ticking the boxes as we go," Cook told City's official website.
"There are a couple of others that we are still working on. As yet, there is nothing more to report - but overall we have had a good transfer window."
Cook has confirmed plans are already in place to replenish City's squad, and over the 12 months that follow.
"Over the next four windows we have got a clear list of players we would like to see at this football club," said Cook.
"We choose not to get into the public space, which allows itself to be speculative, but we clearly know what we're doing - and I think that the fans are going to be excited."