The 24-year-old man was detained by Stockholm District Court on Wednesday after claims a woman was raped on board the ferry between Stockholm and Finland on Saturday.
Eight men in total are accused of being involved in the incident, which allegedly took place on Amorella, an overnight ferry travelling from Stockholm to the Finnish port of Turku, a popular route with a reputation for attracting Swedes and tourists who enjoy partying and can enjoy tax-free alcohol for some of the crossing.
Five men were initially held by Finnish police in Åbo over the weekend alongside another in Mariehamn on the island Åland. Two others were arrested when the ferry returned to Stockholm on Sunday morning.
As the Local reported on Monday four of the suspects were later released while three remained in custody in Finland.
However, on Wednesday Swedish prosecutor Elisabeth Brandt confirmed that the four men who had been released were still being investigated.
"We will ask for their extradition from Finland to Sweden," she said.
The 24-year-old denies the claims, according to Swedish newspaper Aftonbladet.
The suspects are accused of gang raping a woman, 45, who says she was approached in her cabin by a group of men.
According to witnesses speaking to Aftonbladet, the crossing was "messy", with many passengers getting drunk and a fight breaking out at a disco on board the ferry earlier in the night, with staff called to restore order.
Viking Line is one of a number of cruise operators in Scandinavia that has hit the headlines in recent years, following other alleged rapes or sexual assaults.
In 2010, 38 ferry passengers travelling in the region filed rape complaints, double the number in 2006.
Since then, police are understood to have stepped up efforts to share information about attacks among the major ferry companies, to help better identify crime trends.
Tony Öhman, Head of Marine Operations for Viking Line, which has its headquarters in Stockholm, told The Local on Tuesday: "We at Viking Line find the news about the alleged rape very tragic and sad and we are aware that issues concerning sexual assault are a bit of a problem on our cruises, however we work unconditionally with the help from the police force to prevent these crimes from happening."