The Carter Center broke the news Sunday, writing ... "Our co-founder, former U.S. First Lady Rosalynn Carter, passed away this afternoon in Plains, Georgia." In a lengthier statement, they went on to explain that she passed peacefully at home with her family by her side.

The news comes just a couple days after Rosalynn entered hospice care -- which her husband had actually entered as well earlier this year. She'd also recently been diagnosed with dementia.

Rosalynn served the country well while Pres. Carter was in the White House in the '70s -- and her legacy stands as someone with astute political intuition ... not to mention fine work for the American people while in office, especially in regard to mental health awareness.

By most accounts, she was one of the modern First Ladies to truly step into a public role and help shape an administration -- something all First Ladies have continued since.

Of course, her and Jimmy's humanitarian work post-presidency has been widely praised as well -- she and her hubby were the longest married presidential couple in U.S. history.

She's survived by President Carter, their four children and her grandchildren. Rosalynn was 96.