Searches of their homes and vehicles had netted neither weapons nor explosives or other evidence.
“The detainees are being released,” Berlin police spokeswoman Patricia Braemer told AFP.
Interior Minister Horst Seehofer nonetheless praised the detentions Sunday as a prudent preventative measure for a country that has suffered several deadly jihadist attacks.
“We have a very tense security situation,” Seehofer said, adding that Germany faced the continued threat of another attack at any time.
Berlin authorities said Sunday they had detained the men aged 18 to 21 after receiving indications of a possible plot to carry out a “violent crime” at the sports event.
The security services said that on that basis, and after a deadly van rampage on Saturday in the western German city of Münster, they had decided to swoop on the suspects.
The half marathon, which drew 36,000 runners and big crowds of onlookers with 630 police in attendance, went off without incident.
The Die Welt and Der Tagesspiegel newspapers reported that one of the six detainees had links to Tunisian asylum-seeker Anis Amri, who carried out a deadly truck attack on a Berlin Christmas market in December 2016.
Amri had hijacked a truck and murdered its Polish driver before killing another 11 people and wounding dozens more by ploughing the heavy vehicle through the crowd.
He was shot dead by Italian police in Milan four days later while on the run.
Germany, like other European countries, remains a target for Islamist militants, in particular because of its involvement in the coalition fighting the Islamic State group in Iraq and Syria and its deployment in Afghanistan since 2001.
German security services estimate there are around 10,000 Islamist radicals in Germany, some 1,600 of whom are suspected of being potentially violent.