The firm's parent company, Agents' Mutual, will announce a plan to overhaul its corporate structure and raise £50m through a listing tomorrow, Mark Kelinman at Sky News reported.
Investment bank Zeus Capital is said to be working on the inital public offering (IPO).
OnTheMarket launched in 2015, and it has quickly moved up the ranks of the estate agency market, signing up around 5,700 agents across the UK on its portal.
Currently, the firm is owned by its members, including property giants Savills, Chestertons and Knight Frank.
A source told Sky OnTheMarket will seek funds from institutional investors as part of its IPO.
The company's estate agents, which make up its existing shareholders, will first vote over its demutualisation plan.
If the plan to list goes ahead, the funds will be used to accelerate the portal's expansion and the marketing of its operations.
It is working to challenge the "duopoly of Rightmove and Zoopla" by building up a reputation that will attract and retain more estate agents and increase website traffic, sources said.
Existing shareholders will hold onto a majority of OnTheMarket's shares, aligning them with its future performance, according to the source.