Only former Glasgow Warriors fly-half Dan Parks and Wales#' Dan Biggar can claim to have scored more points in the Guinness PRO14 than Keatley, who has amassed 1,524 points over 13 years in the competition.
On the announcement of Keatley, head coach, Danny Wilson, said: "Ian has been a fantastic player over the years for Munster, Connacht and more recently at Benetton.
"His experience will be great from a performance point of view. He will also be invaluable to the development of our young players, including the likes of Ross Thompson, and also enable us to look at Pete [Horne] in his natural position of inside centre."
The fly-half's 212 appearances in the competition are surpassed only by his former Connacht captain John Muldoon, with the Dubliner having also been a part of a Munster squad that reached six semi-finals in his eight seasons at Thomond Park.
Keatley made his Ireland debut in May 2009, scoring 10 points in a victory over Canada. His most recent start for the men in green came in the autumn of 2017, when he slotted the match-winning kick in a 23-20 win over Fiji.
He becomes the first Irish international to represent Glasgow Warriors since James Downey, the centre having pulled on a Glasgow shirt on seven occasions in the 2014/15 campaign.