The annual Patterns of Global Terrorism figures showed that attacks had increased almost fourfold from the 3,129 in 2004.
The US says the increase is because of a change in methodology that devotes more energy to uncovering evidence of attacks on civilians.
But the figures may fuel criticism of the Bush administration's claim that it is winning the "war on terror".
About 3,500 of last year's attacks took place in Iraq, the report said.
Despite this, the State Department says Iraq is not a haven for terrorism.
The al-Qaeda network remains the most dangerous terrorist threat facing the US, despite the arrest of top operational planners in 2005, the State Department said.
Iran is the most active state sponsor of terrorism, the report states, with Cuba, Libya, North Korea, Sudan and Syria also identified as supporting terror.
However, the report praised Libyan and Sudanese efforts to co-operate in the global "war on terror".
Syria was also praised for its attempts to prevent "terrorists" from crossing into Iraq.
Venezuela was criticised by the report for its public denunciations of the US-led "war on terror" and for holding an "ideological affinity" with Colombian rebel groups such as the Revolutionary Armed Forces of Colombia (Farc).