Robin Roberts will begin her medical leave from "Good Morning America" on Friday ... so she can undergo bone marrow transplant surgery.
Earlier this year, Roberts announced she was diagnosed with myelodysplastic syndrome (MDS) ... a disease that attacks the blood and bone marrow.
Roberts says her sister is the bone marrow donor ... explaining, "[She] is a perfect match and she’s been spending a lot of time with me. She’s been taking shots to help her prepare."
Roberts says the procedure is scheduled for Tuesday ... and says she plans to return to work following her recovery.
"I know I’m going to have a job when I come back."
As TMZ previously reported, "GMA" is planning to use a roster of star-studded subs to fill in for Roberts during her absence.
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