The initial price is $20 million cash, with a further $25 million in cash and shares dependent on future performance.
The Heyzap team will be joining Fyber, growing the company's presence in San Francisco.
The key result of the deal will see Fyber and Heyzap's reach extended to almost 550 million active users, when Heyzap's 130 million total is combined with Fyber's 411 million. (Of course, there will be some overlap between the two.)
On the other side of the coin, the combined group will have its network in over 7,600 mobile apps and games.
"This marks an important milestone for Fyber, allowing us to deliver substantially broader global scale and reach for our demand partners, while offering a significantly expanded pool of advertising demand sources for publishers," said Fyber cofounder and COO Janis Zech.
"Both Fyber and Heyzap share core values and a dedicated mission to build the most advanced developer-friendly monetization platform that will fuel the app economy of the future."