She also confirmed their names – which had been rumored after she and Jay-Z filed a trademark for the names.
The picture showed Beyonce and the twins draped in a purple floral sheet, while she wore a blue veil.
It clocked up more than two million likes on Instagram in an hour.
Beyonce wrote: “Sir Carter and Rumi 1 month today”, with a string of emojis of prayer hands and a woman, man, little girl and two babies.
As well as the twins, a boy and a girl, Beyonce and Jay-Z are also parents to 5-year-old daughter, Blue Ivy.
The style of the image, in which Beyonce stands in a garden barefoot in front of a floral archway, echoes the photoshoot she used to announce her pregnancy on the network. That post, in February, became the most-liked in the history of Instagram.
The world had been eagerly awaiting the first glimpse of the babies ever since media reported Beyonce had given birth last month.
However, neither the singer, nor her husband had confirmed any details of the twins until now.
Beyonce’s father, Mathew Knowles tweeted on June 18: “They’re here!” and “Happy birthday to the twins” – but the timing of Beyonce’s post suggest they were actually born on June 14.
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