The Japanese computer games firm posted a net loss of 2bn yen ($24.7m; Â£15.6m) for the six months to 30 September.
This compares with a net profit of 69.5bn yen for the same period in 2009.
Nintendo's sales for the first half of its financial year were down 34% to 363.16bn yen, partly due to lower demand for its Wii console.
The company did not release a net profit figure for its second quarter to 30 September, however its operating profit more than halved to 30.9bn yen.
Nintendo is forecasting that its annual profit will drop to the lowest level in six years, as sales of the Wii console decline for the second year in succession.
The firm has also been forced to delay the launch of the new 3D version of its DS hand-held console in the US until March, meaning it will miss out on key Christmas sales.
Nintendo's weak results were in line with market expectations, as it had already warned at the end of last month that profits would be lower.
The release of its latest figures came after the close of Thursday trading on the Tokyo Stock Exchange. Nintendo shares had closed down 0.7%.
Nintendo has reported a half-year loss after being hit by falling sales and the high value of the yen, which lowers its overseas earnings.