Martina Purdy has been at the corporation for more than 20 years and confirmed her decision to leave in a statement this morning.
She said: "I've been a journalist now for almost 25 years, 15 of them at the BBC.
"It has been an immensely rewarding profession.
"I am especially grateful to the BBC for the opportunities I've been given."
She added: "I know many people will not understand this decision. It is a decision that I have not come to lightly, but it is one that I make with love and great joy. I ask for prayers as I embark on this path with all humility, faith and trust.
"This is a very personal decision. I ask that the media respect my privacy and that of the religious congregation which I am entering, as I face up to the new challenges of my life. I will not be making any further public comment about this matter."
It's not known which order she will be joining.
Director of BBC NI, Peter Johnston, said: "We are extremely sad to be losing Martina from BBC NI's political team.
"She has made a big contribution to our political coverage over many years and she will be greatly missed. I wish her well in the new direction she has chosen for her life."
Martina said she would not be making any further comment or doing any interviews about her decision.