Papini's defense attorney, William Portanova, tells TMZ ... Sherri signed a plea deal Tuesday in which she will plead guilty to counts of lying to a federal officer and mail fraud.

Remember ... Papini made national headlines when she was reported missing in early November 2016 before eventually being found on Thanksgiving about 150 miles away from her Northern California home. She told police she was abducted by 2 Hispanic women and held at gunpoint, even working with an FBI sketch artist to come up with a sketch of her alleged kidnappers.

But, as we reported just last month, Papini was arrested and charged for allegedly making the whole thing up ... with the feds saying she was actually staying with her ex-boyfriend when she claimed she was kidnapped.

Papini was charged with making a false statement and mail fraud, and the judge said she was facing a maximum sentence of 5 years/$250,000 fine and 20 years/$250,000 fine, respectively, if convicted.

Papini's lawyer says they are taking her case "in an entirely new direction" and "everything that has happened before today stops today."

Papini tells TMZ in a statement released through her legal team ... "I am deeply ashamed of myself for my behavior and so sorry for the pain I've caused my family, my friends, all the good people who needlessly suffered because of my story and those who worked so hard to try to help me."

She adds ... "I will work the rest of my life to make amends for what I have done."

While Papini's plea deal has been delivered to prosecutors, those same prosecutors have not yet filed a sentencing memo with their recommended sentence.