The advertisements come a day after Yasir Arafat's top security advisor accused the US of "blackmail" over the 15 October incident that killed US security personnel.
The reward money was splashed over the front page of al-Hayat al-Jadida and was also run by al-Quds newspaper and local radio stations.
"The US Government is offering ... protection of an informant's identity and relocation with their families," said the advert, headlined "Search for Justice."
Three US guards were killed in the roadside explosion as the diplomatic convoy travelled close to the Erez border crossing between Israel and Gaza.
A US embassy spokesman played down the timing of the adverts coming the day after Palestinian leader Arafat's national security advisor, Jibril al-Rajub, said his services were looking for the culprits and should not be pressured.
Diplomat Paul Patin said "the reward has been in effect for a while, soon after the 15 October attack, but was first advertised today. It was just a matter of bureaucratic delays and agreeing on the language."
Al-Rajub hit out at the United States on Wednesday when he said it was "blackmailing" the Palestinians by stopping their involvement in the peace process while awaiting the results of the investigation.
"The Americans have stopped their involvement [in peace efforts], waiting for the results of the investigation," he said.
But in Washington, US State Department spokesman Richard Boucher rejected al-Rajub's claims as "ridiculous".
"Our position is that there needs to be a resolution of the security situation in Gaza, including apprehension of those who are responsible for the killing of US officials there," he said.
At the time of the attack, Washington placed the blame for the convoy bombing squarely on Palestinian leaders for failing to combat violence in the territories.
It also suspended all official and diplomatic visits to the Palestinian territories.
Meanwhile, the head of the Palestinian police escaped an assassination bid when unknown gunmen burst into his headquarters in Gaza City.
"The group of outlaws tried to assassinate the head and founder of the Palestinian police General Ghazi al-Jabali in his office," a police statement said.
Eleven policemen and bodyguards were wounded.