The 35-year-old midfielder joined Mo hours before their weekend 1-1 draw at home to Alloa Athletic after leaving Inverness Caledonian Thistle.
He was pitched straight into the starting line-up and Montrose manager Petrie believes he’ll continue to play a pivotal part as the season goes on.
“I have known Michael since we were at Ross County together, and knew he would be a perfect fit for the side we are building here,” he said.
"We initially spoke at length in the summer, but things didn’t work out then, so I am delighted to have finally got him to put pen to paper on what is a fantastic deal for the club.
"His ability has been proven over many years and he will bring great quality and experience to our dressing room.
“I have often stated how grateful Ross (Campbell, assistant) and I are for the support of the chairman, the CEO and the board who continue to back our vision and plans.
"Securing such a lucrative, marquee signing as this is testament to that commitment and belief and I would once again like to put on record our sincere thanks to John Crawford and all the directors.”
Gardyne is Ross County’s record goalscorer, enjoying a 15-year affiliation with the club before joining Caley at the start of the season.
For the Staggies he has amassed 73 goals in a record 444 appearances and a total of seven medals.
On the move he said: “I’m delighted to have signed and can’t wait to get started. I already know a number of the boys through being pals with Paul Watson and obviously played with Stewart many years ago at County. Hopefully I can add to the group and help them push on in the league this season.”
The club has also confirmed that James Keatings’ loan from Raith Rovers has been extended until June
Liam Callaghan’s goal on 51 minutes helped Montrose to a share of the spoils at Links Park on Saturday, cancelling out Steven Boyd’s first-half opener for the Wasps.
Airdrie’s home win over East Fife cut Mo’s advantage in second position to just a point but the two have now kicked someway clear of the chasing pack in behind.
This weekend Mo travel to leaders Cove Rangers, aiming to eat into their seven point lead.