It happened at an apartment block in the Donegall Quay area of Belfast.
Detective Inspector Richard Thornton said: “Just after 1am, we received a report that two men, armed with knives entered an apartment and demanded money from three male residents.
“They made off with a sum of cash.
“None of the residents was injured.
“We believe that they also made attempts to gain entry to other apartments but without success.
“There was also a report of an attempt to break into business premises at the same location.
“One of the men is described as being 6’ tall, of heavy build with brown hair and wearing a black hoody top.
“The other is said to be 5’ 6” in height, with a reddish face, brown hair and of heavy build.
“He was reported to be wearing a light coloured-top with red on it.
“If you saw either of these men or anyone acting suspiciously in the Donegall Quay or Custom House Square areas of the city between midnight and 2am, please get in touch with detectives at Musgrave on the non-emergency number 101 quoting reference 39 of 19/9/17.
“Alternatively, information can also be passed anonymously via the independent charity Crimestoppers on 0800 555 111.”