The president-elect, who won the all-important Electoral College count, offered no evidence to back his claim.
It comes after Hillary Clinton’s campaign said it would support a vote recount in Wisconsin initiated by Green Party candidate Dr. Jill Stein.
Hillary Clinton won about two million votes more than Mr Trump in the popular vote.
However, Donald Trump surpassed the required 270 electoral votes to win the presidency. This was based on the state-by-state contests.
In a tweet, the president-elect wrote: “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.”
In his follow-up tweets, Donald Trump wrote: “It would have been much easier for me to win the so-called popular vote than in 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)!”
Donald Trump also alleged “serious voter fraud” in Virginia, New Hampshire and California – states won by Hillary Clinton – accusing media of not reporting on that issue.
On November 27, Donald Trump reminded Hillary Clinton that she had already admitted defeat, and published remarks from the presidential debates in which she had urged an acceptance of the poll results.
At the time, Hillary Clinton was reacting to Donald Trump’s refusal to respect the outcome.
The Republican narrowly beat the Democratic candidate in Wisconsin, where a recount of the votes was initiated last week by Jill Stein.
Jill Stein also wants recounts in Michigan and Pennsylvania, citing “statistical anomalies”.
The Green Party nominee reportedly wants to be sure computer hackers did not skew the poll in favor of Donald Trump.
Concerns over possible Russian interference had been expressed in the run-up to the vote.
The US government has said Russian state actors were behind hacks on the Democratic National Committee (DNC), a claim denied by Moscow.
Results would need to be overturned in Wisconsin, Michigan and Pennsylvania to alter the outcome of the November 8 election – something analysts say is highly unlikely.
Hillary Clinton’s campaign has said it will participate in Wisconsin’s recount.
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