News reports quoted a senior Trump administration official as saying that the USA had come to the conclusion, "with a very high level of confidence" that the Lazarus Group, a hacking organisation that works on behalf of Pyongyang, was behind the WannaCry attack.
"We do not make this allegation lightly", he added.
Bossert said that the USA has concrete evidence for the claim.
WannaCry crippled parts of the U.K.'s state-run National Health Service and compromised companies such as FedEx Corp. and Nissan Motor Co.
In May, a ransomware attack known as WannaCry infected computers across the world, encrypting people's files and charging owners hundreds of dollars to recover access to those files.
Trump's homeland security adviser, Tom Bossert, will make the announcement in an op-ed to be published Monday evening and will follow up with a statement Tuesday morning.
"It was cowardly, costly and careless", he wrote. "The attack was widespread and cost billions, and North Korea is directly responsible", wrote Bossert.
It's unclear if the Trump administration will use WannaCry as a way to put more pressure on North Korea via sanctions, as is already the situation with the country's nuclear program.
"So we don't have a lot of room left here to apply pressure to change their behaviour".
American tech giants Microsoft and Facebook acted recently on their own to disrupt the activities of North Korean hackers and prevent the operational execution of ongoing cyber attacks in the USA, the White House said today.
North Korea has been developing cyber capabilities as trade sanctions and a debilitated domestic economy make it hard to invest in conventional military capabilities, said Tom Uren, a visiting fellow at the Australian Strategic Policy Institute's International Cyber Policy Centre. He implored private companies to build up their defenses against North Korea and other "bad actors" in cyberspace, and singled out Microsoft in particular for taking actions last week to "disrupt activities of North Korean hackers", without elaborating on the details. "It gives them something else to bring to the table".
At the time, officials noted some common code with the "Lazarus Group", which officials had previously blamed on North Korea.
North Korean leader Kim Jong Un. Until now, the US government has not publicly stated as much. In addition to the U.K., Australia, Canada and Japan are among countries that agree with the US analysis, Bossert said.