The super-private couple jetted off to Cabo San Lucas from Los Angeles on Friday, Dec. 16.
"It was a quick trip in honor of Katie's birthday," the insider revealed. "It was only them and two pilots on the plane."
The lovebirds spent the weekend hiding out at the luxurious Las Ventanas al Paraíso, in a secluded suite away from the rest of the hotel guests.
"They wanted to be very discreet and had everything delivered to their room and taken care of by a personal butler," the source added.
Katie and Jamie reportedly flew home on Saturday evening, after just one night in paradise.
The duo has kept their alleged relationship very under wraps since word got out that they could be dating back in 2013.
According to the mag, Katie's 10-year-old daughter Suri -- from a previous relationship with Tom Cruise -- stayed back at home during her getaway.
The "All We Had" actress' actual birthday is on Sunday, Dec. 18.
Happy birthday, Katie!