In the works for close to a month, Klemmer was able to sign a nine-month deal with the Dragons on Friday as the NRL's contract merry-go-round kicked into gear.
The 31-year-old's move came after Dragons winger Mikaele Ravalawa was released to South Sydney for the rest of this season, creating the roster spot for Klemmer.
Ravalawa will move to the Rabbitohs immediately in a bid to help end their backline shortage, before returning to the Dragons in 2026.
The Dragons had been left in recruitment limbo for the past two months, while desperate to add an extra middle forward to their squad.
With their 30-man roster complete, the Dragons' plans for 2025 were hit hard when prop Francis Molo began an indefinite leave of absence from the club in January.
Ultimately that vacancy was created by Ravalawa's move on Friday, with the Fijian international to be available for the Rabbitohs from round three.
"I've said publicly we are looking to bolster our middle forward stocks for this season so David is a great pick-up for us," Dragons coach Shane Flanagan said.
"He's a different body shape to what we've got in our forward pack currently and also brings some great experience at Test, Origin and club level."
Klemmer will immediately link up with the Dragons, but their bye next week means he won't play NRL with the club until the following round against Melbourne.
His move also means the Tigers will move closer to having enough space to offer former Penrith winger Taylan May a lifeline.
May is free to play in the NRL again after domestic-violence charges were dropped on Monday, after the wife he was accused of punching did not show up to court.
The 23-year-old has not played since last May, sidelined while fighting the charges and sacked by Penrith.
May was spotted at the Tigers' opening-round win over Newcastle, with his older brother Terrell playing his first game for the club.
The Tigers are in desperate need of backs with a likely decision to be made on the potential medical retirement of Justin Olam later this month.
Tigers captain Apisai Koroisau said earlier this week his former Penrith teammate would not have an issue regaining his form.
"He has been out there so much before, it's not like he is trying to reach that level for the first time," Koroisau said.
"It's hard to come by these kind of guys. He works so hard in the backfield, and you need that from your wingers these days. And he is so quick."