A new advertising and digital campaign will be launched in the summer.
Quiet Storm won the account following a pitch against two undisclosed rivals.
Trevor Robinson OBE, Creative Director of Quiet Storm says: “THTC is a forward-thinking and eco-friendly brand with very similar values to our own.”
The company has been making T-shirts from a mixture of organic, carbon-neutral cotton and hemp, using organic dyes for a decade.
The T-shirts feature political slogans such as ’George Bush & Son, Family Butcher’ alongside original graphics.
The brand includes celebrities Damon Albarn, Benjamin Zephaniah and Woody Harrelson among its fans.
THTC aims to highlight the benefits of hemp farming without promoting cannabis smoking.