"This platform will go beyond measurement to motivate and drive improvement and make the road to personal transformation fun and social," Costolo said in a Tweet.
Though the entrepreneur did not give the name of the startup, he stated that it will provide an outlet for wellness professionals to share their expertise and guidance.
The move marks a new phase in Costolo's career. Last June, he announced that he was stepping down as Twitter's CEO and would be replaced by Twitter co-founder Jack Dorsey.
The tech executive also tweeted that he will be joining Index Ventures as a partner. The venture capital firm based in San Francisco and London has invested in companies such as SoundCloud, Etsy, King Digital Entertainment and Pure Storage.
"Having the opportunity to work with the great team at Index while also embarking on my own new venture makes the beginning of 2016 a very special time for me," said Costolo. "I can't wait to dive in."
-- Reuters contributed to this report.