In a series of tweets, Costolo didn't give details on what types of companies he'll focus on at Index, just saying he'll invest in entrepreneurs and work with existing portfolio companies. Costolo said he's gotten to know the firm though a friend, partner Danny Rimer.
In an email to CNBC on Tuesday morning, Costolo said his years running Twitter through its IPO helped him discover an interest in fitness.
Costolo wouldn't reveal the name or launch plans for his fitness start-up, but said in a tweet Tuesday that he and co-founder Bryan Oki are looking to reimagine the path to personal fitness.
In an email to CNBC, Costolo added: "Over the course of the last couple of years working with Bryan (cofounder) in various group settings, I observed both my personal development and increased physical energy as a leader. We plan to bring this process and capability to a much larger audience. This process improves productivity, it increases energy levels, and it builds camaraderie among teams — all things I've lived through personally."
Without providing any details of what the fitness platform will be like, Costolo did delineate what it's not: "This isn't about creating software for one fitness community, such as Crossfit or yoga or Soulcycle. It's a platform for everyone."
And Costolo said his experience at CES just a few weeks ago influenced his plans to focus on a seamless, unified fitness platform. "One sees the entire fitness ecosystem becoming increasingly large but also fragmented and confusing and it's not clear the ecosystem is putting the consumer first," Costolo told CNBC. "We are going to change that and bring all these disparate pieces together in a way that is simple, fun, and social."