Mr Trudeau visited James Smith Cree Nation in the Canadian province of Saskatchewan three months after the September attacks.
He paid his respects at the burial site of victims and met local leaders.
Mr Trudeau also announced C$62.5m (£38.7m) in funding for the First Nations community.
A bulk of the money will help build a new wellness centre in James Smith Cree Nation and repurpose an existing treatment lodge for substance use to support the mental health of survivors and residents, Mr Trudeau’s office said in a press release.
“Today, I heard first-hand the pain, sorrow, anger and grief that people here in the community of James Smith Cree Nation are feeling,” Mr Trudeau said after his visit.
The stabbing spree, which unfolded on 4 September over Canada’s Labour Day weekend, shocked the small indigenous community of nearly 3,500 members.
It also sparked a massive manhunt for brothers Myles and Damien Sanderson, who were then believed to be jointly responsible for the attacks.
Police later said they suspected Myles Sanderson was acting alone and had killed 11 people, including his brother Damien and nine others from James Smith Cree Nation. Another person was killed in Weldon, a quiet farming town, of about 200 people, nearby.
Myles Sanderson died shortly after being arrested on 7 September. The cause of death has yet to be determined.
In October, Saskatchewan Royal Canadian Mounted Police (RCMP) said that “we will never really know why” the incident took place.
The stabbings have sparked calls from local leaders for the creation of their own First Nations police force, which they said would be better positioned to protect the community in collaboration with the RCMP.
Chief Wally Burns of James Smith Cree Nation said Mr Trudeau’s visit gave his community confidence that it can move forward on its goals, including self-administered policing.