The state department said the reward for information leading to the arrest or conviction of anyone involved in the terrorist attack in Benghazi had been on offer since January but had not been publicised.
The men died on 11 September when militants, believed to have ties to al-Qaida affiliates, attacked a US diplomatic compound and a nearby CIA annex in the eastern city of Benghazi on the anniversary of the 2001 terrorist attacks on the US.
The deaths triggered political warfare on Capitol Hill, with Republicans accusing President Barack Obama's administration of telling shifting stories about who was behind the attacks. Democrats accused Republicans of politicising a tragedy.
The state department said the secretary of state, John Kerry, had confirmed that the US government had offered the reward – part of the department's "rewards for justice" programme – in a letter to legislators on Friday.
Kerry disclosed the reward in a letter to Representative Michael McCaul, a Republican who chairs the House of Representatives homeland security committee and who had written to the secretary of state asking why the state department had not offered one.
The state department said in a statement that the reward had not been publicly advertised due to "security issues and sensitivities surrounding the investigation".