"I’m a little concerned about United Technologies and Raytheon," Trump told CNBC.
Trump used the aerospace industry as an example of how mergers can adversely affect business.
"We used to have many plane companies... they have all merged. Now we have very few," Trump said." It's hard to negotiate when you have two companies and sometimes you get one bid."
Asked whether he would have a problem with the merger, Trump replied: "Only if they have the same products. That would be the thing that bothers me most."
On Sunday, Raytheon and United Technologies announced they agreed to merge the defence company with United Technologies' aerospace business.
The two companies dismissed Trump’s concern of reduced competition, claiming they have very little overlap in their product lines that could spark anti-trust concerns, media reported.
The merger remains subject to regulatory approvals and is expected to close in the first half of 2020.