The 24-year-old, who played a part in all three of the province's Heineken Cup triumphs, had also been linked with London Irish but has committed himself to Leinster for the next 12 months.
Fitzgerald, who will miss the first half of the season having undergone neck surgery, has won 23 caps for his country and toured South Africa with the Lions in 2009.
"We are delighted that Luke has committed to the province for another year," said coach Joe Schmidt.
"Luke is an outstanding talent and he has played a major role in Leinster's recent success."
The European champions have also bolstered their squad with the signings of lock Quinn Roux and utility back Andrew Goodman on one-year deals.
Roux, 21, joins from Western Province and recently made his Super 15 debut for the Stormers.
Goodman, 29, makes the switch from Tasman Makos in New Zealand.
Schmidt said: "Quinn is a physically imposing forward with a burgeoning reputation in South Africa and he will add further competitiveness to the second row alongside Leo Cullen, Damian Browne, Devin Toner and Mark Flanagan as well as our other recent signing Tom Denton.
"Andrew is a quality player and has been a consistent place-kicker for a number of years. Both players are determined to make a positive impression over the forthcoming season and we are very pleased to have secured their services."
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