Former MLB pitcher Brian Matusz -- best known for his years on the mound with the Baltimore Orioles -- died from an apparent overdose ... TMZ Sports has confirmed.

Our sources tell us Matusz was found dead at his Phoenix, Ariz. residence on Jan. 6 ... with a white substance around his mouth and drug paraphernalia -- including a straw, lighter and aluminum foil -- near his right hand.

We're told it was Matusz's mother, Elizabeth, who discovered him ... after she had grown concerned about him due to comments he made about feeling like he had nothing left to live for.

Our sources say Matusz had a history of abusing substances ... though he had recently gotten clean.

Matusz -- the fourth overall pick in the 2008 draft -- pitched in the MLB from 2009 through 2016 ... logging playing time with the Orioles and Cubs. He had signed with the Diamondbacks in 2017 but was released a short time later.

Matusz's death rocked the baseball world ... with the Orioles saying in a statement, "Brian was beloved throughout Birdland, and his passion for baseball and our community was unmatched. He dedicated his time to connecting with any fan he could, was a cherished teammate, and always had a smile on his face."

Matusz was just 37 years old.

RIP