Rightmove currently receives 25m visitors every month, and wants to solidify this position, and secure a greater search visibility.
I Spy Search will be tasked with creating and managing paid search campaigns and natural search optimisation, which includes keyword research, site analysis and link building.
Stuart Chung, market intelligence manager at Rightmove, said: "Our members recognise the benefit of ever greater search visibility. I Spy took this feedback onboard and developed an extensive and strategically well thought-through plan to meet the needs of Rightmove and our customers."
Chris Whitelaw, chief executive of I Spy, said: "Rightmove has always been a truly pioneering brand, it's a real honour to have won this business. Search has been placed at the heart of Rightmove's growth which is further recognition of the growing value of the medium."
Rightmove.co.uk offers details of properties currently for sale or rent in the UK and overseas, and also offers properties for holiday rental, both at home and abroad. It also publishes the monthly House Price Index.
I-Spy poached Yahoo!'s search marketing director Nick Jones as its managing director earlier this year.
The agency claims to have pioneered a unique integrated search marketing system, since it opened its doors in 2006.