Three of its units were found to have falsified data for products supplied to nearly 300 companies in the aerospace, car and electric power industries.
No safety issues have been reported.
The discovery is the latest in a string of scandals tarnishing the image of corporate Japan and its reputation for precision manufacturing.
Kobe Steel, Mitsubishi Motors, Nissan and Subaru all recently admitted to manipulating data. Several of them saw the quality lapses stretch on for decades.
Mitsubishi Materials said it first became aware of the “misconduct” at its divisions in February and opened an investigation in the wake of the Kobe Steel scandal.
Mitsubishi Materials has a 45% share in a copper tube joint venture with Kobe Steel, including the Hatano plant that is at the centre of the Kobe scandal.
On Thursday Mitsubishi Materials apologised for the manipulation and promised to strengthen its quality control procedures to stop the problem being repeated.
The firm said the impact of the quality scandal on its financial results “are currently unknown”. It is scheduled to hold a press conference later on Friday.