The step comes after TikTok's Chinese owner, ByteDance, said it had finalised a deal to set up a majority US-owned joint venture that will secure US data, to avoid a US ban on the short video app used by more than 200 million Americans.
"Following TikTok's sale to a Trump-aligned business group, our office has received reports, and independently confirmed instances, of suppressed content critical of President Trump," Newsom's office said on X on Monday, without elaborating.
"Gavin Newsom is launching a review of this conduct and is calling on the California Department of Justice to determine whether it violates California law," it added.
The White House and TikTok did not immediately respond to a request for comment.
Newsom, a Democrat, and Trump, a Republican, have long been critical of each other.
Last week's TikTok deal was a milestone for the firm after years of battles with the US government over Washington's concerns about risks to national security and privacy under Trump and former President Joe Biden.
ByteDance said TikTok USDS Joint Venture LLC would secure US user data, apps and algorithms through data privacy and cybersecurity measures, in a deal praised by Trump.
With more than 16 million followers on his personal TikTok account, Trump credited the app with helping him win the 2024 election.
The deal provides for American and global investors to hold 80.1 per cent of the venture while ByteDance will own 19.9 per cent.
Each of the joint venture's three managing investors, cloud computing giant Oracle, private equity group Silver Lake and Abu Dhabi-based investment firm MGX, will hold a stake of 15 per cent.
The US and Chinese governments had signed off on the deal, a White House official said.
