The button, mounted upon a large wooden box, sat beside two phones on Donald Trump’s desk.
And when Mr Trump pressed the red button, a butler would emerge from another room, carrying a bottle of Diet Coke on a silver platter.
Substantial changes were made in the Oval Office in the hours between Mr Trump’s departure and Mr Biden’s arrival – and the Diet Coke button has not survived the transition.
The change was noticed by reported Tom Newton Dunn of The Times.
“When (Tim Shipman) and I interviewed Donald Trump in 2019, we became fascinated by what the little red button did,” Newton Dunn wrote on Twitter.
“Eventually Trump pressed it, and a butler swiftly brought in a Diet Coke on a silver platter. It’s gone now.”
Photos of Mr Biden signing executive orders show nothing has changed of the president’s desk-space, save for the button.
Neither Mr Biden or Mr Trump drink alcohol, but their beverage of choice is different.
Beside Mr Biden in the photo is a cup of tea or coffee.
Meanwhile, Mr Trump is reported to have consumed up to a dozen Diet Cokes a day.
Various other changes to the Oval Office have already been noted. A bust of Mr Trump’s father Fred, a wealthy real estate developer, has been replaced with one of labour activist Cesar Chavez.
And a portrait of President Andrew Jackson has been replaced with one of Benjamin Franklin.
Mr Jackson has been a celebrated president in the past, but his role in the genocide of Native Americans has made him a controversial figure in recent years.
A beige rug has been replaced with a dark blue one bearing the presidential seal.
A bust of Winston Churchill has been removed, and a moon rock from Apollo 17 is on display. A bust of civil rights icon Rosa Parks has also been added.
The desk itself is staying put. The desk was made from the timbers of the British Arctic exploration ship the HMS Resolute, and was a gift from Queen Victoria to President Rutherford B Hayes in 1880.
The 590kg piece of furniture has been shifted around the White House but has been in the Oval Office for most of the last 40 years.
A table immediately behind the Resolute desk holds photos of Mr Biden’s family.