Allan Avery, Father of 'Making A Murderer' Subject Steven Avery, Dead at 89

Allan Avery -- the father of Steven Avery, whose legal troubles were documented in the hit Netflix documentary "Making A Murderer" -- has died ... according to the family's lawyer.

Attorney Kathleen Zellner revealed the sad news via social media Sunday ... noting that he was Steven's biggest supporter and advocate -- who believed in his innocence, 100%.

Allan appeared in the documentary ... the first season of which came out in 2015. The doc dove into Steven's wrongful sexual assault conviction and subsequent 18-year incarceration -- as well as his 2007 conviction for the murder of freelance photographer Teresa Halbach.

The documentary brought renewed attention to the case ... and Allan was at the forefront of pushing for Steven's release.

He also worried that he and his wife, Dolores, wouldn't live long enough to see Steven released from prison ... telling the documentary crew at one point, "My greatest worry is that me and Mama don't pass away before Stevie gets out. 'Cause it can happen. When we get to see Steven, he gives us all a big hug."

Dolores passed away in 2021 at the age of 83. Steven turned 64 in July.

Allan was 89.