The London-based agency will aid Rebel Minds' digital content production and develop a digital marketing and PR strategy. Semaphore says it is hopeful of being retained after the initial project concludes.
The Rebel Minds website claims that in the past year more than 1.1 million people have seen the digital products and services it has designed for various brands.
Rebel Minds founder Jonathan Fren said the agency had shown "a deep understanding of our needs as a rapid growth digital company".
Semaphore founding partner Liam Fay-Fright said: "Across all of its work, Rebel Minds celebrates excellence, from the beauty of its design and concept work to the robustness of its builds. It produces great work, and we’re looking forward to helping it create even more."
Fay-Fright co-founded the agency in March alongside Aaron Cole – the pair had both previously worked for ad agency Mother. The same month it was appointed to a Europe-wide brief with mattress company Eve Sleep.