A member of Ulster's 1999 Heineken Cup-winning squad, 'Fitzy' left Ravenhill at the end of the 2002/03 season for France where he spent two seasons with Castres Olympique. The 2005/06 season marked the beginning of his second spell with Ulster.
Fitzpatrick, who made his Ireland senior debut aged 24 against South Africa in June 1998, has 26 Test caps to his name - the last recorded against Italy in Limerick prior to the 2003 Rugby World Cup.
The Chicester-born forward, who is a member of the ERC Elite club having won over 50 Heineken Cup caps, was very happy to put pen to paper on a new deal.
Fitzpatrick said: "I am very glad to be extending my contract with Ulster. The support that the team has here at Ravenhill is fabulous.
"Ulster Rugby has been a major part of my rugby life and I am excited about the direction that the team is taking under the guidance of Matt Williams and I am very much looking forward to the next two seasons."
Williams himself, who is preparing his squad for the Magners League visit of Llanelli Scarlets on Friday, said: "I am delighted that Justin has signed with Ulster again. He is currently playing the best rugby of his career. The side will be further strengthened by his great experience and his leadership over the next two seasons."