China spent more than $145bn on its military last year, amassing drones, warships, jets, missiles and cyber weapons, according to the Pentagon.
Beijing had previously said its military spending for 2013 was $119.5bn.
The Pentagon investigation used 2013 prices and exchange rates in a report to Congress which produced a figure 21% higher than China's official records.
The US Department of Defence admitted the difficulty in trying to produce an accurate assessment of China's spending due to "poor accounting transparency and incomplete transition from a command economy".
China's defence ministry rejected the Pentagon report, saying it was "resolutely opposed" to its conclusions.
"Year after year the United States issues this so-called report on Military and Security Developments in China, making preposterous criticisms of China's normal defence and military building, exaggerating the 'China military threat', which is totally wrong," it said.
"As for the detailed contents of this year's US report, we are currently assessing it, and will react further, depending on the situation."
The report also warned of China's cyber security capabilities and follows the United States' indictment of five Chinese military personnel for economic espionage. In an unprecedented step, the FBI published 'Wanted' posters for the Chinese officials, who were accused of hacking into American solar, nuclear and metal companies.
"China is using its... capability to support intelligence collection against the U.S. diplomatic, economic, and defence industrial base sectors that support US national defence program," it said.
The report follows US Defence Secretary Chuck Hagel's strongly-worded rebuke of China's territorial ambitions in the region, after a high profile clash with Vietnam in the South China Sea.
China claims almost all of the energy-rich waters for itself, rejecting competing claims from the Philippines, Vietnam, Taiwan, Brunei and Malaysia. The dispute with Vietnam erupted in May after China deployed a massive oil rig in waters claimed by the Vietnamese.
The clash was followed by massive anti-China protests in Vietnam as rioters attacked Taiwanese factories, mistakenly thinking they were Chinese.