The British and Irish Lions fly-half has completed terms on a lucrative IRFU and Leinster central contract, in a clear boost to Ireland manager Joe Schmidt's Rugby World Cup 2015 preparations.
Sexton had been widely tipped to return to Ireland after his second year in France, and has now made good on the rumours.
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"It's a choice for my family, my international career and my extended family in Ireland," said Sexton. "I have a great rapport with everyone here, it was a very hard decision to make.
"I am pleased to be rejoining Leinster, the club where I started my professional career."