Foreign ministry spokesperson Mao Ning said the aircraft had deviated from its route, having been blown off course.
A similar balloon was shot down in US airspace by military jets on Saturday amid allegations that it was being used for surveillance.
China has denied accusations of spying, saying it was monitoring the weather.
The incident has led to a diplomatic row between Washington and Beijing.
On Friday - before fighter jets brought down the balloon at the weekend - US military officials said a second Chinese balloon had been spotted over Latin America.
On Monday, China admitted an aircraft had "accidentally entered Latin American and Caribbean airspace".
Ms Mao told reporters the second balloon had "deviated greatly" from its intended route, citing the aircraft's "limited manoeuvrability" and the weather conditions.
"The unmanned airship in question that came from China is of a civilian nature and used for flight tests," she added.
"China is a responsible country and has always strictly abided by international law in order to inform and properly deal with all parties concerned, without posing any threat to any country."
At the weekend, Colombia's air force said an object with "characteristics similar to those of a balloon" had been detected on 3 February in the country's airspace at above 55,000ft.
Colombia said it had followed the object until it left the airspace, adding that it did not represent a threat to national security.
Meanwhile, work by US Navy divers continues to recover the wreckage of the surveillance balloon that was shot down off the coast of South Carolina on Saturday.
US President Joe Biden first approved the plan to bring down the balloon on Wednesday, but decided to wait until it was over water so as not to put people on the ground at risk.
The US believes the balloon was being used to monitor sensitive military sites.
Adm Mike Mullen, former chair of the US Joint Chiefs of Staff, rejected China's suggestion it might have blown off course, saying it was manoeuvrable because "it has propellers on it".
"This was not an accident. This was deliberate. It was intelligence," he added.
Relations between China and the US have been strained by the incident, with the Pentagon calling it an "unacceptable violation" of its sovereignty. A planned trip by Secretary of State Antony Blinken to China was cancelled as a result of the row.
China has lodged a formal complaint with the US embassy in Beijing over the incident.