Toyota and Isuzu agreed to work together on diesel engine development in 2006, as well as explore other collaborative projects.
As part of the agreement Toyota purchased 50 million shares in Isuzu, equating at the time to about 5.9 per cent of the automaker's stock.
According to the companies, "once-in-a-century changes" and movement in the "market environment", most likely the widespread abandonment of diesel technology in favour of partial or full electrification, have caused most of the joint development projects to be suspended.
Isuzu and Toyota have also seen "little specific progress" in other collaborative works.
Although the two companies are going their separate ways, they still want to be friends and "maintain their strong relationship". They say they will continue to work together on "ongoing joint development projects related to basic technologies", although no specifics have been provided.
The two Japanese automakers say they "remain open to the possibility of future collaboration".