The bill skipped the committee stage, going straight to an Upper House vote. Opposition parties accused the ruling bloc of steamrolling the legislative process.
The law punishes all members of a group involved in planning a serious crime if authorities find at least one of them preparing for it. The government says the law is necessary to beef up security before Tokyo hosts the Olympics and Paralympics in 2020.
Opposition parties had tried to block a vote on the bill with a non-confidence motion, but that was voted down.
Critics say the law could suppress intellectual freedom and be applied arbitrarily by law enforcement agencies. They also say there was not enough deliberation on it.
The enactment came just days before the end of the current Diet session.
The public has mixed opinions about the bill's passage:
"It's necessary to stop crimes before they happen. But I'm not happy with how the bill was enacted. I think the ruling parties just pushed it through without enough debate."
"I think there is some danger in criminalizing acts at the plotting stage, but I hope there will be a drop in crime."
"I don't support the legislation and feel it doesn't reflect people's opinions. I think it's scary that you can be taken in based on an assumption that you were plotting a crime when you actually may have no intention to commit one."
Following the bill's enactment, Japanese Prime Minister Shinzo Abe said the law will protect people's lives and property.
"Japan is preparing to host the Tokyo Olympics and Paralympics in 3 years. Now that the legislation is enacted, we hope to ratify the UN Convention against Transnational Organized Crime as soon as possible and work together with the international community to prevent terrorism."
Meanwhile, the main opposition Democratic Party leader Renho lashed out at how the process played out.
"It was an ultimate act of steamrolling. The LDP, Komeito and Nippon Ishin refused to debate the legislation in an Upper House Committee. The chamber is known as a house of deliberation. I cannot understand why they would tarnish that," she said.
NHK World's Minori Takao is joined by senior commentator Takashi Ichinose in the studio.
Takao: Why was this bill rushed through the Upper house of the Diet?
Ichinose: That's exactly what opposition parties are asking. Critics say the bill had a lot of issues that still need to be clarified. They claim there was not enough time devoted to this issue. In fact, it was given minimal debate in the Lower and Upper house before being enacted this morning.
Some people are speculating that the government wanted to pass the bill before the election campaign of the Tokyo Metropolitan Assembly starts next week.
However, the government said the bill was being passed quickly because the legislative session wraps up on this Sunday and the new measures are essential to ensure national security.
Takao: So why is this law so contentious?
Ichinose: Critics say the law is too broad and far reaching. They are concerned it gives authorities too much power and that it will infringe on civil liberties and human rights. The law covers 277 crimes, some that do not involve terrorism. The opposition sees it as a watered-down version of the conspiracy bill which was introduced three times but never went to vote since 2003.
The UN Special Rapporteur on the right to privacy has even weighed in.
He wrote a letter to Prime Minister Abe expressing 'serious concern' that it may fundamentally affect public freedoms.
Public opinion is also divided. A poll we conducted on Monday shows those for and against the bill are nearly even, but nearly 40% of people are undecided -- which goes to show that people do not know enough and are unsure.
Takao: What happens now?
Ichinose: Now that the bill has become law, the government is going to implement it. Officials say they will ensure authorities do not abuse it. At the same time, the government needs to explain to the public why the new law won't infringe on their rights. But it is going to be a hard sell.