The resignation will be effective on 31 December this year. Sensis chairman, Bruce Akhurst, said although the company is sad to see him go, but said Day is "leaving the business in an extremely strong position".
"Sensis is delivering strong and sustainable revenue and profit growth, and the business is executing a sound diversification strategy that both protects and enhances the core directory business," he said.
Day, who took on the role in October 1999, has "led the company on an impressive growth journey" from being a largely print-based company to a "multi-channel, search and advertising business," Akhurst said.
"It is a testament to all Sensis staff that the business is in such a strong position," said Akhurst. "I am very confident that the existing leadership team, most of whom have been with Sensis for several years, will continue to develop Sensis into an even stronger company over the years ahead."
Telstra said it has now commenced the process of recruiting a new chief executive officer for the company.