The value of the US dollar's short position totaled $441.7 million from a net long position of position of $4.57 billion the previous week. It's the first net short position on the dollar since the week of Sept. 6, 2011.
To be short a currency is to bet it will fall in value, while being long is a view its value will rise.
The Reuters calculation for the aggregate US dollar position is derived from net positions of International Monetary Market speculators in the yen, euro, British pound, Swiss franc and Canadian and Australian dollars.
Euro net shorts further declined to 101,561 contracts this week, while net longs on the yen rose to 21,556 contracts.
Euro sentiment has improved markedly the last few weeks on expectations of further peripheral bond buying by the European Central Bank in efforts to keep interest rates in debt-plagued countries lower.
The euro zone's common currency ended the month of August on a positive note, notching gains of 2.3 percent after losses of 2.8 percent the previous month.
The US dollar index, on the other hand, closed this month down 1.7 percent following gains of 1.2 percent in July.
Further weighing on the US dollar was an increase in net longs on the Canadian dollar to 60,936 contracts and a decline in short bets on the Swiss franc to 11,461 contracts.