The South African international is currently playing in an ODI series against Zimbabwe alongside Kyle Abbott, who he will replace as Hampshire's overseas player for the Championship matches against Kent at The Ageas Bowl and Glamorgan in Cardiff.
Tahir, who took 124 wickets at 25.24 in 27 first division matches for Hampshire from 2008-11, including 7-66 on debut against Lancashire at Old Trafford, will make his division two debut when Kent visit West End on September 15.
Director of cricket Giles White said: "We're keent to bring Immy back, there are couple of things to sort out including his visa, but he's very fond of the club and is looking forward to returning.
"He's a matchwinner, especially at this time of year."
Tahir, 35, has made all 15 of his Test appearances since he last played for Hampshire (taking 40 wickets at 47.17). He also has 18 ODI and 12 T20 caps.
Hampshire will play their next Championship match, at home to Leicestershire on August 31, without an overseas player.