The 24-year-old became Charlton's second loan signing on Friday following the arrival of defender Dan Seaborne from Southampton.
Addicks boss Chris Powell, desperate to bolster his team who are hovering three places above the drop zone, told cafc.com: "Jonsson is a centre midfielder who keeps things simple and ticking over.
"He wins his tackles and has got a lot of energy and gets about the pitch.
"I just felt I needed to boost the midfield area. We need to have players that have games behind them and Eggert comes into this category."
The Icelandic star moved to Wolves from Heart of Midlothian at the end of last season after making 153 appearances for the club.