Essent Trading, an existing Syzygy client, appointed the agency's subsidiary, Unique Digital, following a competitive pitch.
Syzygy bought Unique Digital last April and is part of its strategy to offer new services to existing clients. Unique has since won clients including Sky and Harvey Nichols.
Syzygy is roughly 30%-owned by Sir Martin Sorrell's WPP group and is billed as a WPP associate company.
The full-service interactive agency has 230 staff across Europe, with more than 125 based in the UK.