The EU's executive commission said it is concerned that the deal would entrench Google's position in the online ad market by "increasing the already vast amount of data" that the company could use to personalize ads.
EU competition commissioner Margrethe Vestager said the "investigation aims to ensure that control by Google over data collected through wearable devices as a result of the transaction does not distort competition."
Google agreed to buy Fitbit in November for $2.1 billion. Privacy and consumer groups have called on authorities to block the deal, citing privacy and antitrust concerns.
The EU noted that it has the power to block the decision if it is not satisfied with Google's solutions to its concerns.
A final decision is due by 9 December, it said.