Trump won the Electoral College and thus the White House, but the Democratic nominee leads him in the popular vote by about two million ballots.
"In addition to winning the Electoral College in a landslide, I won the popular vote if you deduct the millions of people who voted illegally," Trump tweeted.
"It would have been much easier for me to win the so-called popular vote than the Electoral College in that I would only campaign in 3 or 4- states instead of the 15 states that I visited. I would have won even more easily and convincingly (but smaller states are forgotten)!" he added.
This is the first time he has alleged voter fraud in his own victory and there is no evidence of any widespread voter fraud.
Trump could be referencing a series of fake stories on conspiracy websites that said he actually beat Clinton in the popular vote count. Trump's transition team did not return requests for comment Sunday afternoon.
He later added: "Serious voter fraud in Virginia, New Hampshire and California - so why isn't the media reporting on this? Serious bias - big problem!"
Trump has been railing over the weekend against a recount effort led by the Green Party, that he has dubbed a "scam." Green Party officials filed for a recount in Wisconsin on Friday
after reports of possible voting discrepancies in areas that used paper ballots versus those where electronic voting took place.
Wisconsin Green Party co-chairman George Martin said the party is seeking a "reconciliation of paper records" -- a request that could go further than a simple recount, possibly spurring an investigation into the integrity of Wisconsin's voting system. "This is a process, a first step to examine whether our electoral democracy is working," Martin said.
Both the Clinton campaign and the White House have said they see no evidence that any voting systems were hacked, although the Clinton campaign said
Saturday it will take part in the recounts, joining with Stein, to ensure the recount is "fair to all sides."
Earlier Sunday, Trump launched one of his trademark tweetstorms, reiterating his previous criticism of the recount effort, taking aim at Clinton.
"Hillary Clinton conceded the election when she called me just prior to the victory speech and after the results were in. Nothing will change," he tweeted.
And then he quoted Clinton's own concession statement, writing: " 'Trump is going to be our President. We owe him an open mind and the chance to lead.' So much time and money will be spent - same result! Sad."
Trump on Electoral College
Trump has vacillated in his support for the Electoral College. In 2012, he tweeted: "The electoral college is a disaster for a democracy."
Since his election, he's been more laudatory.
"The Electoral College is actually genius in that it brings all states, including the smaller ones, into play. Campaigning is much different!" he tweeted about a week after Election Day.