Zaniewska will work with Nissan, Adidas and GSK, among others and report to chief strategy officer Zaid Al-Zaidy.
Zaniewska will also focus on breakthrough creative ideas, alongside the agency's other strategy directors, Toto Ellis and David Frymann.
Prior to joining Iris Digital, Zaniewska worked at AKQA London where she was senior strategic planner.
Zaniewska said: "This is a fantastic time to be joining TBWA\London and I'm looking forward to working with the agency’s incredible talent to develop further transformational ideas."
Al-Zaidy said: "Kasia has a rare and brilliantly varied set of strategic skills, which makes her a perfect fit for TBWA\London."