The former Leeds United boss is heading to Belgian football.
Thomas Christiansen is back in management around a-year-and-a-half after he left Leeds United.
The Dane had been without a club since his Elland Road exit, which came just eight months after he joined the Whites.
But he was unveiled as the new boss of Royale Union Saint-Gilloise on Monday afternoon.
Alex Muzio, who is chairman of the Belgian second-tier side, told the club's official website: "We have taken our time to make the right decision and a coach who shares our vision for the long term. Of all the profiles, Thomas stood out for his ambition, his determination and his experience. I am delighted that a coach with Thomas Christiansen's CV joins us and shares our desire to evolve and grow the club."
Christiansen led Leeds to the top of the Championship by autumn the season before last, but the Whites' form soon began to stutter.
By the time of his sacking in early February, Leeds were 10th - and went on to finish 13th under his successor, Paul Heckingbottom.
Heckingbottom failed to make the new campaign, however, and was replaced by the incumbent Whites boss, Marcelo Bielsa.
As well as Leeds, Christiansen has managed the Cypriot sides, AEK Larnaca and APOEL Nicosia.
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