Rob Glaser, the company's founder and chairman, has returned to the CEO job to fill in on an interim basis, RealNetworks said.
Nielsen is not leaving to take a position elsewhere, the company said in a Tuesday.
RealNetworks has struggled financially in recent months. The company reported a net loss of US$17 million in the first quarter of 2012, following a loss for the full year in . In January, it sold $120 million in video software and patents to chip maker Intel.
"I come back to RealNetworks with a sense of great urgency to set the company on a long-term path to success," Glaser said in the statement.
Glaser founded RealNetworks in 1994. Prior to that he had been vice president of multimedia and consumer systems at Microsoft.
The IDG News Service