Erik Asphaug of the University of California, Santa Cruz, said he believes the landscape of our moon contains evidence of the smaller moon from when the pair collided, The Daily Telegraph reported.
"The second moon would have lasted for only a few million years; then it would have collided with the moon to leave the one large body we see today," he told the British newspaper.
The smaller "twin" would have have been about one-thirtieth the size of our moon, he said.
"It would have orbited Earth at the same speed and distance and just got slowly sucked in until they hit and then coalesced," he said.
Most scientists have supported the theory the moon was once part of the Earth that was thrown off after a collision with another body.
Asphaug will present his theory of two moons at a conference about the moon to be held by Britain's Royal Society in September, the Telegraph said.