Qatar hosts a big US military base housing the headquarters of USA air forces in the Middle East.
Qatari Defense Minister Khalid Al-Attiyah met with his USA counterpart, James Mattis, to sign the agreement Wednesday.
The Pentagon said in a statement that the deal "will give Qatar a state-of-the-art capability and increase security cooperation and interoperability between the United States and Qatar".
The F-15 agreement would lead to "closer strategic collaboration in our fight to counter violent extremism and promote peace and stability in our region and beyond", according to the defence ministry.
"This agreement underscores the longstanding commitment of the State of Qatar in jointly working with our friends and allies in the USA", a media quoted al-Attiyah as saying in the statement.
"Given the high volume of food production in Iran and the current restrictions placed on Qatar, we are fully ready to provide the Arab country with its required food supplies", he added.
The US has been seeking to reassure one of its allies in the region since President Donald Trump tweets appeared to back theSaudi-led boycott on Qatar on charges that it sponsors terrorism and aligns itself with Iran, claims rebutted by Doha.
Five Eastern countries - Bahrain, Egypt, Saudi Arabia, the United Arab Emirates, and Yemen - broke off ties with Qatar, hoping to isolate the country politically and economically, punishing Qatar over its alleged support for terrorism.
U.S. agrees with KSA/UAE that Qatar is a "state-sponsor of terrorism", and then sells Qatar $12 billion in arms.
But coming on the heels of the unprecedented spat between Doha and its neighbors, the deal nearly compartmentalizes US-Qatari defense relations, he added. Boeing Co is the prime contractor on the fighter jet sale to the Middle East nation and it wasn't immediately clear if the two deals were one and the same. Its regional feeder flights in Saudi Arabia and the UAE also have been cut off. Al Udeid Air Base is home to more than 11,000 United States and coalition troops.
President Trump took the Saudi side - a position quickly countered by the Pentagon, which has praised Qatar for hosting the U.S. airbase and for its "enduring commitment to regional security".
Despite a neutral stance being maintained by secretary of state Rex Tillerson and defence secretary James Mattis, Trump has contradicted that by also accusing Qatar of being a "high-level" funder of terrorism.