After initially joining on a two-year deal, Rennie was due to be out of contract at the end of the season but will now remain in his post until 2020.
Speaking to BBC Scotland ahead of Saturday's match against Benetton, the New Zealander said: "I'm going to be here for at least another year and a half - possibly longer.
"It's been really good. We're happy, and my wife is happy, that's the key.
"If I'm not here who is going to be running the show and contracting? It gives players clarity over who is going to be here."
In his first season Rennie guided Glasgow to the PRO14 semi-finals, and they currently top Conference A.