Morris played eight times for Leinster in the Guinness PRO12 before departing for English Premiership side Leicester Tigers in 2011.
The utility back featured 75 times for the Tigers and scored 18 tries in his five seasons at the club, which included scoring a try in their victorious 2012/13 Premiership final.
Speaking to Leinster's website, the 27-year-old said: "After five enjoyable years in England, I am delighted to return to my home province, Leinster.
"I know the club well and it has fantastic players, staff and supporters. It is one of the biggest clubs in Europe and I am very much looking forward to being part of such a great set-up again."
Morris has played for the Emerging Ireland team on three occasions and has scored two tries in six outings for Ireland Under-20s.
Leinster head coach Leo Cullen added: "Niall was unlucky with injuries in his last season in England but has performed at a very high level for the Tigers in both the Premiership and in the Champions Cup.
"He brings good experience to our back three, an area that is already very competitive and we look forward to seeing Niall in action on a pitch soon."
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