The fitness device marker sees the tech giant as "an ideal partner" to expand after the business was making a loss and struggling with waning demand with other companies entering the market.
Co-founder and chief executive James Park said: "Google is an ideal partner to advance our mission.
"With Google's resources and global platform, Fitbit will be able to accelerate innovation in the wearables category, scale faster and make health even more accessible to everyone."
Fitbit - which was founded 12 years go - has said its "health in wellness" data won't be used by Google adverts amid concerns from regulators, and the company has vowed to continue its strong privacy protections.
However, in a note cited by Reuters, Craig Hallum analysts warned: "The deep health and fitness data, coupled with the 28 million active users on the Fitbit platform, offer a tremendous value."
Subject to approval by the board and regulators, the acquisition is expected to be confirmed and completed in 2020.