"Don Jr. and Kimberly are seeing each other, and are having a great time", one source told the gossip outlet.
On Sunday evening, the two were seen arriving at U.S. Ambassador to Germany Richard Grenell's party, where they mingled with guests before leaving together to attend a dinner at Harry Cipriani on Manhattan's Fifth Avenue, reported Market Watch.
Guilfoyle, 49, grew up in the Mission District of San Francisco and graduated from Mercy High School, an all-girls Catholic school. Guilfoyle was previously married to California Lt. Gov. Gavin Newsom.More news: Trump's meddling is likely to work against US World Cup bid
The news comes on the heels of the president's oldest son being "dad-shamed" for posting a video on Instagram of his 3-year-old daughter Chloe not wearing a shirt.
Guilfoyle is also a former model who has reportedly worked for lingerie brand Victoria's Secret.
"Her portfolio is stunning", one SF Gate reporter wrote in a 2001 profile when Guilfoyle was still engaged to Newsom.
Guilfoyle was married for several years to Eric Villency, a design guru from New York City and the father of their son Ronan Anthony, born on October 4, 2006.More news: Israel Attacks Syria Less Than an Hour After Trump Speech
Multiple sources have confirmed that the pair have been dating for "several weeks", according to Page Six.
She was also linked to former White House communications director Anthony Scaramucci past year when he briefly separated from his wife, Deidre Ball.
Guilfoyle is now the co-host of Fox News Channel's The Five, which airs weekdays at 5:00 p.m. ET. In addition to The Five, Guilfoyle regularly appears on other FNC shows as a legal analyst.
Guilfoyle revealed last May that she was in talks with the Trump administration to become the White House press secretary following Sean Spicer's departure.More news: Dr Mahathir says 'confident of winning' Malaysia election