The move reflects the resurgence of the sector in the UK and the agency's renewed faith in chasing internet-based clients.
Mantra launched in 2000 as a tech agency but has since diversified into consumer and corporate work.
Fundamental is headed by Mantra MD Debbie Wosskow, who will initially be assisted by two PROs: Sophie Raine, who has joined from consumer and sports PR specialist Paul Ridley Ltd, and Mantra executive Nicky Savage.
Wosskow and Savage will divide their time between Mantra and Fundamental, while Raine will focus on Fundamental.
Wosskow said: "Fundamental is for those tech start-ups that are in the very early stages of their development. We will not be looking to work on a retainer, only on a project basis from one day to six months at the most."
But she added: "Eventually we hope they will graduate to longer contracts with Mantra."
Shares in British tech firms rose this year to their highest level in four years, and the increasing number of broadband users, plus the success of websites such as MySpace, bode well for online brands, according to analysts.
"This move highlights the revival of the sector," said Wosskow. "Mantra started life working with tech start-ups and Fundamental will be able to work with the new generation."
Fundamental has one client, a networking club for VCs and dotcom entrepreneurs called Second Chance Tuesday, which it won after a two-way pitch.