They claimed that the woman that has recently appeared with US President Donald Trump, looks “nothing like her”.
Others said that the decoy is using a fake nose and big sunglasses to disguise herself.
The online chatter began on Friday when Mrs Trump appeared next to her husband on the White House lawn while he talked about hurricane-ravaged Puerto Rico.
Mrs Trump appeared wearing large sunglasses.
As he spoke to reporters, President Trump referred to “my wife, Melania, who happens to be right here”, a comment which was seized on as evidence that she was not the real thing.
Elizabeth Peterson LaSalle posted a video of Mrs Trump online and a picture of her with her sunglasses off.
She wrote: “This is just a bit weird. Compare the two photos of this “woman” who is supposed to Melania Trump.
“Look at the hair – but especially compare the lips and the nose in these two photos.
“I don’t know who this woman was standing with Trump in the video but it is not Melania.”