Though the official opening of the event started at 10 a.m., crowds of burger fans waited in line at Columbus Coffee Company in Upper Thomson from early morning as the event only took place for one day, The Straits Times reported.
Three kinds of burgers were available at the pop-up event - the famous "double-double" burger that cost S$11 (US$8.13), cheeseburgers for S$7 and hamburgers at S$5.
Many netizens expressed their surprise with the long queues for In-N-Out on social media.
"No way a burger can be so good as to get you out of bed at 3:30 a.m. right ... right???," one online user commented.
"I would never queue at 3 a.m. for a burger," wrote another.
"Queue for hours, finished the burgers in minutes," another said.
In 2022, the chain drew a massive crowd of burger lovers in Vietnam after it opened a pop-up event in HCMC’s District 1.
In-N-Out Burger was first opened in 1948 by Canadian-born businessman Harry Snyder and his wife Esther.
The burger chain is yet to establish an outlet outside of the U.S. but has opened pop-up locations across the world in locations such as Sydney, Tokyo and Singapore.
This is the fourth time In-N-Out has held a pop-up in Singapore.