It was not immediately clear exactly how the alert was linked to the terror group.
Threats were made against the central Hauptbahnhof station and the Pasing station west of the city, Police Chief Hubertus Andrae said.
The stations were evacuated on Thursday evening and service stopped for around eight hours.
Authorities told the public additional officers were being called in to find possible suspects. On Friday, 100 additional officers were still on duty, Andrae said.
The information police received referenced five to seven possible attackers and included personal information for some of them. Those details included names and referred to both Syrian and Iraqi nationals, the chief said.
Initial police checks of the information turned up nothing, and no one has been detained, Andrea said.
Another country's intelligence agency reported the plot and the ISIS connection to German federal police, Bavarian Interior Minister Joachim Herrmann told journalists. Andrae would not confirm reports that the intelligence that sparked the alert came from U.S. and French intelligence.
Early Friday, police allowed both train stations to reopen, they announced in a tweet.
Hermann said there were no concrete indications that a terror attack could take place in Munich in the coming days. "Right now the threat level is the same as it has been ... since the Paris attacks,' he said.
"We all know there is a danger of attacks in all of Europe and also in Germany and here in Bavaria. But we have no specific evidence as to imminent attacks today or tomorrow in any specific places," he added.
Europe has been on edge since the November 13 terrorist attacks in Paris
that killed at least 130 people
. Recent weeks have brought arrests of suspected terrorists in Belgium, France, Austria and other nations.