A national police spokesman told the BBC that the victims were shot dead during an ambush in the Enga province over the weekend.
Although there has always been violence in the Highlands, these killings are thought to be the deadliest in recent memory.
Illegal firearms have increased the death toll from fights and fueled a cycle of violence.
At first, the authorities reported that at least 64 individuals had died. But according to later reports, they had tallied incorrectly and reduced the number to 26. The police in Papua New Guinea have been contacted by the BBC to confirm this.
At the area close to Wabag, which is about 600 kilometers (373 miles) northwest of the capital Port Moresby, police began gathering bodies.
The Australian Broadcasting Corporation (ABC) was informed by Royal Papua New Guinea Constabulary Acting Supt George Kakas, "This is by far the largest [killing] I've seen in Enga, maybe in all of Highlands as well."
We're all emotionally exhausted and heartbroken. It's quite difficult to understand."
According to media reports, police were sent explicit images and videos purported to be from the scene that showed bodies being placed onto a truck.
Growing tribal strife, frequently centered on land and income distribution, resulted in a three-month lockdown in Enga last July, during which the authorities enforced movement restrictions and a curfew.
After grisly footage of three dead men went viral online in August of last year, the violence garnered international attention.
Prior to the ambush, there had been indications, according to Governor Peter Ipatas, that combat might break out once more.
He claimed that the security forces were ultimately responsible for maintaining peace because up to 17 tribes were involved in the most recent escalation.
"From a provincial perspective, we knew this fight was going to be on and we [alerted] the security forces last week to make sure they took appropriate action to ensure this didn't occur."
General security is still PNG's top priority. A state of emergency was proclaimed by the government last month following widespread looting and riots that claimed at least fifteen lives.
One of the nation's closest allies, Australia, described the news of the executions as "very disturbing".
In a Monday radio interview, Prime Minister Anthony Albanese stated, "We're providing considerable support, particularly for training police officers and for security in Papua New Guinea."