The new office on Great Suffolk Street adds to the existing one in Putney for Just Health, which employs 35 people and recently made three new hires.
The company said it would be announcing new client wins over the next few months.
MD Kirsty Mearns said: "As we start our tenth year, this expansion is symbolic of how far Just has come and I’m extremely proud to launch this new creative space.
"Our flexible working policies allow our employees to choose how and where they like to work, giving them the freedom to produce the best results for our clients. The Southwark office adds another element to that.
"We’re also looking forward to offering the more central location as an alternative meeting space to our Putney HQ, and will be working hard over the coming months to ensure it really feels like a second home for our creative, curious, collaborative agency."