The investigation will look at whether the big tech firms are stifling competition, according to a statement.
DOJ antitrust regulators are "reviewing whether and how market-leading online platforms have achieved market power and are engaging in practices that have reduced competition, stifled innovation, or otherwise harmed consumers," according to a statement.
"Without the discipline of meaningful market-based competition, digital platforms may act in ways that are not responsive to consumer demands," said Assistant Attorney General Makan Delrahim of the Antitrust Division in the statement.
The department intends to "assess the competitive conditions in the online marketplace in an objective and fair-minded manner," according to the statement. "If violations of law are identified, the Department will proceed appropriately to seek redress," it concluded.
In a note sent late Tuesday, Wedbush analyst Dan Ives wrote that news of the DOJ investigation is a "major shot across the bow" amid increasing noise concerning the regulation of Big Tech.
The antitrust probe will represent a near-term overhang on the tech stocks as regulators examine their business models, he added. However, he also speculated that the worst case scenario would be a tweaking of the companies' business models along with potential fines, rather than a forced breakup of any of the companies involved.
"We believe that Congress will investigate claims of anticompetitive behavior, but ultimately, we expect a 'no harm, no foul' outcome on these FAANG names when the 202 area code comes knocking although this news could add some headline risk to these names in the near-term," Ives wrote.
Shares of Alphabet, Amazon and Facebook all fell 1% or more in after-hours trading, while Apple shares lost 0.5%.
Facebook is expected to announce a $5 billion settlement Wednesday related to privacy violations in the Cambridge Analytica scandal.