The Englishman originally moved to the Highlands in the summer of 2011 and went on to make 39 appearances and score five goals before returning south after just one year.
But Caley Thistle boss John Hughes was happy to welcome him back to Inverness after freeing up room in his budget by releasing Curtis Allen and Torbjorn Agdestein.
He said: "We are delighted to secure the services of Greg, who will add to the strength in depth within our squad.
"We faced tough competition to get him on board from down south and from clubs within the SPFL but we are very pleased he has chosen to join us.
"He will go straight in to this weekend's squad for the game in Aberdeen, subject to international clearance."
Tansey added: "I am very happy to return to Inverness, I know the lads and I thoroughly enjoyed my last spell at the club.
"We have some exciting games coming up and I look forward to helping the club continue their magnificent progress over the past couple of years."