"He’s been receiving information about the irregularities and about the illegal votes," Conway said in an interview with ABC.
The adviser added that Kansas Secretary of State Kris Kobach alerted Trump on illegal voting.
On Sunday, Trump claimed in posts on his Twitter account that he would be the winner of the popular vote in the November 8 election if the tally discounted millions of allegedly illegal votes.
White House spokesman Josh Earnest said on Monday there was no evidence to back up Trump’s claim.
According to the latest tracking data, Democratic nominee Hillary Clinton won the popular vote by 2.5 million votes.