According to the BBC, the bus was carrying Vietnamese tourists, as well as two Egyptians - a driver and a rep from the tourist company.
The bomb went off at about 18:15 (16:45 GMT) on Maryoutiya Street in Giza's Haram district, the interior ministry says.
The white tourist bus could be seen with its windows shattered and surrounded by soot-covered debris, AFP reported.
President Abdel Fahah al Sisi is closely following all of Marioteya bus terrorist attack updates along with the ministers and security bodies according to Prime Minister Mostafa Madbouly on Friday.
"We stand with all Egyptians in the fight against terrorism and support the Egyptian government in bringing the perpetrators of this attack to justice", State Department spokesperson Robert Palladino said.
"A group of terrorists were planning to carry out a series of aggressive attacks targeting state institutions, particularly economic ones, as well as tourism. and Christian places of worship", the ministry statement said.
New Russian weapon 'renders missile defences useless'
The Kremlin said it successfully hit a designated practice target on the Kura shooting range on Kamchatka, 3,700 miles away. Reprinted with the permission of Radio Free Europe/Radio Liberty, 1201 Connecticut Ave NW, Ste 400, Washington DC 20036.
The last deadly attack on foreign tourists in Egypt was in July 2017, when two Germans were stabbed to death in the Red Sea resort of Hurghada.
Tourism is a mainstay of Egypt's economy.
Egypt has been seeking to lure tourists back by touting new archaeological discoveries and bolstering security around archaeological sites and in airports.
Tourism has slowly started picking up.
Egypt has battled Islamic extremists for years in the Sinai Peninsula in an insurgency that has spilled over to the mainland at times, hitting minority Christians or tourists.
The Interior Ministry said that "this comes as a continuation of the ministry's efforts in chasing terrorist elements involved in the implementation of hostile operations seeking to destabilize the country's security".