The PropertyFinder Group includes propertyfinder.com, which has over 2m unique users per month, as well as HotProperty.co.uk and UKPropertyshop.co.uk.
Under the deal, Zoopla.co.uk will take over and integrate propertyfinder.com and the other websites into its business, benefiting from PropertyFinder Group's revenues of almost £7m a year.
One of the most established players in online property search, propertyfinder.com was acquired by News International and REA Group for £14.3m in late 2005. HotProperty.co.uk and UKPropertyShop.co.uk were added to the group last year.
This latest buy comes after Zoopla.co.uk's acquisition of ThinkProperty.com from Guardian Media Group last month.
Alex Chesterman, founder and CEO of Zoopla.co.uk, said: "This is a transformational deal both for our business and for the online property landscape in the UK.
"Our organic growth over the past year has been exceptional but this acquisition is a great fit for us and will accelerate us significantly towards our goal of becoming the leading residential property resource in the UK, providing consumers with useful information and tools to help them make better-informed property decisions."
Zoopla.co.uk, which launched early last year, raised almost £4m of further funding at the start of 2009. Its backers include Atlas Venture and Octopus Ventures, as well as a number of angel investors.