Arrest made in Turkey as Germany mourns 10 suicide bombing victims
The arrest came as police carried out numerous raids across the country against cells linked to the Islamic State militant group, which has been blamed for the attack.
Germany's Foreign Ministry confirmed all 10 dead in the blast - which went off near some of Istanbul's most historic buildings - were German citizens.
Regional officials in Germany say the victims were mostly pensioners on holiday in Turkey.
Interior Minister Efkan Ala staged a joint press conference in Istanbul with his German counterpart, Thomas de Maiziere, announcing the arrest, which he said took place in the hours after the attack on Tuesday.
Investigations into the blast were widening, with a picture of the attacker beginning to emerge. Germany has sent special investigators from its federal police to help Turkish security and intelligence services.
Ala said authorities in Istanbul had taken the fingerprints of the attacker a week before the blast. The Dogan news agency cites sources as saying he had registered with immigration authorities as a Syrian refugee and was identified by a finger found at the blast site.
Nabil Fadli, 27, was born in Saudi Arabia, but left with his family in 1996, the Saudi-owned al-Hayat newspaper reported. He had Syrian citizenship and apparently crossed into Turkey recently.
Turkey hosts approximately 2 million Syrian refugees, some of whom have expressed concern they could face blowback as a result of the bombing.
The Turkish interior minister also reported 11 injured remain in hospital, including two Germans in critical condition.
"Based on the current state of the investigation, there is no evidence the attack was directed specifically against Germans," de Maiziere said, calling this an "attack against humanity."
Turkey - which has come under criticism from people saying it has been slow to tackle Islamic State - has arrested more than 3,300 people in operations against the extremist group since last year, including 220 people in the week prior to the attack, Ala said.
Earlier, the Dogan news agency reported that, since the attack, Turkish police arrested nine alleged members of Islamic State in coastal Izmir province and southern Antalya province. Among them were three Russian citizens.
Turkey's economy is heavily dependent on its tourism sector and there is some concern the attack could have a negative effect. Germans make up the largest group of visitors to Turkey.
Hotels in key tourist hotspots have begun to report cancellations. Some tourists in Istanbul said they were avoiding crowded areas.
Tourism from Russia, the second largest supplier of visitors, is already in a tailspin following the decline in the Russian economy and worsening relations between Moscow and Ankara after Turkey downed a Russian jet near the Syrian border in November.
Turkish Prime Minister Ahmet Davutoglu, accompanied by his wife, visited the scene of the suicide bombing and laid red carnations at the site.
No group has yet claimed responsibility for the Istanbul attack. Turkish authorities blamed two major suicide bomb attacks last year on Islamic State militants, though the Sunni extremist group did not claim responsibility. The group routinely claims responsibility for attacks in other parts of the world.
The carnage took place in the Sultanahmet area, home to the famed Blue Mosque and Hagia Sophia museum - relics from Byzantium and the Ottoman Empire.
Turkey last year stepped up efforts to tackle Islamic State, which holds territory in neighbouring Iraq and Syria, after a period in which the government was criticized for being slow to counter the militia and close its porous Syrian border.
The newspaper Hurriyet said the country's intelligence services had warned the security forces twice in the weeks before the attack about the possibility of a suicide attack targeting foreigners.
German Chancellor Angela Merkel, who condemned the "murderous act" in Istanbul, has spoken with Turkish President Recep Tayyip Erdogan by telephone.
