Kamara has signed for an undisclosed fee after penning a three-year contract at the Reebok Stadium. Meanwhile, De Ridder has joined on a one-month loan deal from the Saints.
Trotters boss Dougie Freedman told the club's official website on Kamara's arrival: "I didn't think I would have any chance of getting Medo.
"He's got experience of the Champions League and he's played in some top games.
"He's very hungry to succeed in England and he's choosing Bolton because he feels we're in the very same situation as him."
The duo join defender Cian Bolger in signing for Bolton on transfer deadline day.