She will lead strategy for key clients such as the BBC and Marks & Spencer, as well as "diversifying the agency’s output".
Djaba will report to Jon Sharpe, the chief executive, on an interim basis until a new chief strategy officer has been appointed.
She joined Droga5 in 2013, when she was known as Tamsin Davies, and left last summer. Djaba previously worked on Girl Effect, which aims to help girls in poverty, at the Nike Foundation.
Sharpe, RKCR/Y&R’s former chief innovation officer, took over from Ben Kay as the chief executive in December. He has since brought managing partners, strategy partners and creative partners together to lead accounts.
Djaba was the head of innovation at Fallon London between 2001 and 2011. She worked on the agency’s Cadbury, BBC, Skoda, Sony and Orange accounts. Djaba began her career in 1998 at Abbott Mead Vickers BBDO.
Sharpe said: "Tamsin has created some of the most innovative and cutting-edge creative work in recent years.
"We have had a busy start to the year bringing in new clients such as the Premier League and Chanel – and, with her impressive and diverse skillset, we look forward to Tamsin helping us build the businesses of both new and existing clients."