With 61,000 properties, franchise group Belvoir is the UK’s second largest letting agency and will actively promote the portal with digital and branch-based marketing activity.
Belvoir has a network of 300 franchised lettings and estate agents branded Belvoir!, Northwood and Newton Fallowell.
Dorian Gonsalves, Belvoir’s chief executive, said: "We have for some time believed that the property portals landscape was ripe for disruption.
“OnTheMarket is the first serious contender for several years to be injecting genuine competition in the interests of agents and property-searchers alike.
“It is agents who provide the portals with the majority of their content and income.
“It is also agents who serve their clients and property-searchers on the ground in selling, letting, buying and renting property.”
Ian Springett, OnThe Market's chief executive, added: "While we have already been working with some individual offices within the Belvoir Group, the decision by the group overall to support OnTheMarket is a strong testament to our success to date in increasing our agent base.”