Elsewhere, Microsoft's Xbox 360 outsold the PS3 – but even combined, the two consoles’ sales couldn’t match Wii.
The total US market, including software and hardware, was down seven per cent to $1.27 billion, compared to $1.35 billion for the previous month.
"Compared to August, nearly all hardware systems realised an increase in unit sales in September," commented Anita Frazier of NPD.
"The price reduction on the Xbox 360 helped deliver a month-over-month unit sales increase for that platform of 78 per cent."
NPD hardware unit sales for September read:
* Nintendo Wii - 687,000
* Nintendo DS - 537,000
* Xbox 360 - 347,000
* Sony PlayStation Portable - 238,000
* Sony PlayStation 3 - 232,000
* Sony PlayStation 2 - 173,000