The governor of Red Sea province, where the resort of Hurghada is located, said two "foreign residents" of the city were killed in the attack, a cabinet statement said.
Egypt's official Al-Ahram newspaper reported on its website that two Ukrainian female tourists died in the stabbing and that four others were injured.
But Kiev's ambassador to Cairo told the privately owned Ukrainian broadcaster Channel 112 that no Ukrainians had been killed.
The interior ministry said in a statement that the attacker, who had swum ashore, was arrested and was being questioned.
"We don't know his motives yet, he could be crazy or perturbed -- it's too early to tell," a senior interior ministry official told AFP.
In January 2016, three tourists in Hurghada were wounded in a stabbing assault by two assailants with apparent Islamic State group (IS) sympathies.
There was no immediate claim of responsibility for Friday's stabbing.
Hurghada is one of Egypt's most popular beach resorts, especially with Ukrainians and European tourists.
Security has been boosted in resorts around the country, as the tourism industry provides Egypt with much-needed revenues.
An IS bombing of a Russian airliner carrying holidaymakers from a south Sinai resort in 2015 killed all 224 people on board and decimated the country's tourism sector.
Russia suspended all flights to Egypt in response and has yet to resume them.
IS has been waging a deadly insurgency based in the north of the Sinai Peninsula that has killed hundreds of policemen and soldiers.
Policemen shot dead
Also on Friday, unknown assailants shot dead five policemen south of Cairo, in the latest of a series of attacks targeting the country's security forces.
The ministry said three gunmen opened fire on a police car and then fled, killing a non-commissioned officer, three conscripts and a police employee.
The attack took place near Badrasheen, a town some 20 kilometres (12 miles) from Cairo, where militants have also targeted police in the past.
As with the beach stabbing, there has not yet been any claim of responsibility for the attack.
The killings came as police and the army said they were closing in on militants and jihadists following a spate of deadly attacks in the Nile Valley and the Sinai Peninsula.
Egypt has struggled to quell IS jihadists based in the Sinai and smaller militant groups in the mainland since the military overthrew Islamist president Mohamed Morsi in 2013 and cracked down on his supporters.
IS jihadists killed at least 21 soldiers in restive north Sinai on July 7, the same day as the militant Hasam group claimed responsibility for shooting dead an officer with Egypt's secret police in an attack north of the capital.
While smaller groups such as Hasam have mostly targeted policemen and government officials, IS has also attacked foreign tourists and Egypt's Coptic Christian minority.
Dozens of Christians have been killed in church bombings and shootings since last December in attacks claimed by IS.
The jihadists have threatened to carry out further attacks on Christians, who make up about 10 percent of Egypt's 90 million people.
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