In a statement it said the acquisition of Mediasyndicator and in particular the business and assets under the Sportsyndicator name, would extend its network by over 40 sports websites, 280 million ad impressions and over 8 million unique users.
Rightster will also gain around ten sales and support staff from Mediasyndicator, representing "the majority" of the team.
Charlie Muirhead, the founder and chief executive of Rightster, said: "We've gained a lot of sports sites for our network and a team that has good relationships with those sites and a great track record of selling sports display advertising.
"Rightster can provide all of those sites with a constant flow of sports video content from the 400 rights holders we represent as well as our expertise in selling pre-roll advertising against that content."
He added that Mediasyndicator offered "compelling synergies with our existing business" and that Rightster would look to "engage and monetise Mediasyndicator's relationships across online sites whether they be accessed through desktops, tablets, smartphones or connected TVs".
The acquisition is Rightster's second of 2013, following its purchase of Preview Networks in February, and comes ahead of its planned IPO on the London Stock Exchange Alternative Investment Market (AIM) later this year.