The platform, which allows businesses to borrow sums starting at £250,000, launched in September 2014 and is backed by Hampden Group.
Work on the campaign began this month after a competitive four-way pitch earlier in the year.
There was no direct incumbent. The win sits alongside Lewis' other 'alternative finance' clients, which include Octopus Investments and PayPal.
Ruth Jones, deputy MD UK of Lewis, said: "UK SMEs are the growth engine of the economy, yet traditional financing is hard to secure for the majority. ArchOver’s p2p lending platform is disruptive as it provides a new platform for borrowing, while giving individuals a safer way to realise better returns. We are excited to be part of the team that gets to share the ArchOver story."
ArchOver is Financial Conduct Authority-authorised and has lent more than £37m and generated average returns of 7.26 per cent to investors since its inception.
Last week, Lewis gave James Smee the role of director of EMEA digital marketing, three years on from his agency Purestone being acquired by the global firm. Purestone's co-founder left the business.