The 22-year-old joined the Saints from Leeds Carnegie last summer and went on to make a major impact in his first season at Franklin's Gardens, making 35 appearances and leading the side on several occasions.
He was named the club's Young Player of the Season and while Saints have not revealed the length of the new deal, Clark is delighted to be staying with Jim Mallinder's side.
"I certainly hope that the fans will see me improve in the next few years," said Clark, who has started all three Premiership fixtures so far this term.
"I definitely feel like Northampton is the right club to be at, especially at this stage of my career. The coaches are very hands on and talking to us as players. They're focused on the team winning and players getting better as individuals and achieving the most they can.
"Moving to a club that is in a strong position off and on the field has really helped me plan for the future. I've been able to focus on the rugby and I definitely feel like I've improved as a player.
"I've been here for a season and we've enjoyed a fair bit of success, even though we didn't finish the last campaign in the way we wanted. Hopefully there's more to come from me personally and from us as a squad."
Director of rugby Mallinder added: "Calum certainly has a bright future ahead of him and I'm delighted that he has chosen that it will be at the Saints.
"We believe that we are a club that is on the up and securing the commitment of players like Calum is very important for us."
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