BMB will create a brand campaign to coincide with Father's Day on 17 June.
The agency won the business after a pitch. It is the first creative agency hire by the retailer, which previously developed work in-house.
Notonthehighstreet.com was launched in 2006 by the entrepreneurs Holly Tucker and Sophie Cornish, and wants to ramp up its marketing activity, increasing its adspend to more than £1 million.
The retailer offers bespoke products and gifts created by more than 2,500 independent designers and manufacturers.
Mark Hodson, the chief marketing officer of notonthehighstreet.com, said: "We are delighted to be working with BMB. They demonstrated a complete understanding of what our business is all about. We are excited to work to build brand awareness which will help support the 2,500 UK creative small businesses who sell with us."