The West Tyrone MLA will join Alex Attwood at the negotiating table while Dolores Kelly will take Mr McDevitt's place at the Policing Board.
The former South Belfast MLA stepped down last week after acknowledging that he had not declared some payments.
SDLP leader Alasdair McDonnell said: "Joe will provide a strong voice for ordinary people whose lives are so negatively affected by on-going conflict and contentious parades."
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