Liquid will also develop direct marketing materials for a new investment product, details of which have yet to be disclosed.
Octopus launched in 2000, and is driving the retail market for venture capital trusts (VCTs), which it wants to make as well-known and popular an investment product as the ISA. Octopus claims that VCTs are no more risky than ISAs and have better tax benefits.
Helga Slater, Octopus's marketing director, said Liquid's culture and approach to business was similar to theirs.
She said: "Liquid impressed us with their passion for brand-driven communication, and their record for a young company is impressive.
"Through aggressive, eye-catching marketing and brand communications we want to educate consumers and raise awareness of the benefits of investments such as VCTs."
Olly Raeburn, managing partner at Liquid, said: "We're excited about this brief as it gives us an opportunity to work with a young company at an important stage in its development."
Liquid's other clients include Virgin Mobile and Marks & Spencer Financial Services.
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