Austrian Interior Minister Gerhard Karner said an 18-year-old was taken into custody on Thursday night in Vienna after allegedly being in contact with the main suspect.
Karner announced the arrest during an unrelated news conference on Friday.
Investigators are scrutinising the "networks" of the suspects, the Austrian interior ministry told the Associated Press on Friday, adding investigators had turned to evaluating physical and electronic evidence.
The 19-year-old main suspect and a 17-year-old were arrested Tuesday, while a 15-year-old was also interrogated but was not arrested.
The foiled attack was planned for Thursday or Friday, Interior Minister Gerhard Karner said. (AP PHOTO)
Officials said on Thursday afternoon that no additional suspects were being sought, and did not immediately offer further details on Friday.
Authorities say the plot appeared to have been inspired by the Islamic State group and al-Qaeda.
Investigators found bomb-making materials at the main suspect's home.
Officials say that teenager confessed to planning to "kill as many people as possible outside the concert venue" using knives or homemade explosives.
Officials say the main suspect and the 18-year-old pledged "oaths of allegiance" to the Islamic State group.
The 18-year-old "had been in contact with the main perpetrator but is not directly connected to the attack plans", the minister said.
Three sold-out concerts were cancelled Wednesday because of the plot, devastating Swifties from across the globe.
Taylor Swift fans gathered in Vienna to commiserate after her shows were cancelled. (AP PHOTO)
Many of them had dropped thousands of euros on travel and lodging in Austria's expensive capital city to attend the Eras Tour shows at the Ernst Happel Stadium.
Concert organisers in Austria said they had expected up to 65,000 fans inside the stadium at each concert and as many as 30,000 onlookers outside, where authorities said the suspects planned to strike.
The foiled attack was planned for Thursday or Friday, according to Karner.
Austrian Chancellor Karl Nehammer defended the decision to cancel the concerts, saying the arrests took place too close to the shows, scheduled for Thursday, Friday and Saturday.
"I understand very well that those who wanted to experience the concert live are very sad," Nehammer told a news conference.
"Mums and dads are looking after their daughters and sons, who were full of enthusiasm and anticipation for this concert.
"But it's also important that in such serious moments as now, it's inevitable that safety comes first."
Authorities said the suspects planned to strike at crowds outside the Ernst Happel Stadium. (AP PHOTO)
Concert organiser Barracuda Music said all tickets would be refunded.
Swift is scheduled to perform at London's Wembley stadium in five concerts between August 15 and 20 to close the European leg of her record-setting Eras Tour.
London Mayor Sadiq Khan said while he understood Vienna's reasons for cancelling, "we're going to carry on".
Khan said the capital's authorities were prepared for shows there following lessons learned from a 2017 attack by a suicide bomber at an Ariana Grande concert in Manchester, England, that killed 22 people.
In July, an attacker in England killed three girls and wounded 10 people in a knife attack during a Taylor Swift-themed dance and yoga class.
Swift at the time said she was ''completely in shock'' over the violence.
She has not spoken publicly about the plot or cancelled shows, but Taylor Nation, an Instagram page widely believed to be run by her team, reposted the announcement from Barracuda Music in a "story", which is only visible for 24 hours.
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