Charlotte, who is affectionately known as Lottie to her family, can be seen grinning at the camera in a pretty floral, puff-sleeved, button-front navy dress from children's clothing company Rachel Riley, in the photo taken by her mum Kate.
After the picture was posted, many fans noted that the royal had an uncanny resemblance to another family member – her father Prince William.
WATCH: Kate Middleton and Princess Charlotte are hair twins
"She looks like her Daddy!" wrote one, while another added: "She is William's twin!!!!"
A third posted: "So like her daddy," while a fourth said: "Wow, she took William's whole face."
A fifth fan saw a bit of both her parents in the picture, saying: "She has her daddy's face and her mama's hair except blonder."
Others noted just how much the little girl looked like her great-grandmother, the Queen.
Charlotte turned six on Sunday
Charlotte is the middle child and only daughter of William and Kate. She was born at 08:34 in May 2015 in the Lindo Wing of St Mary's Hospital in Paddington, London – the same hospital where Kate gave birth to her two sons, Prince George and Prince Louis.
Her name was announced as Charlotte Elizabeth Diana a couple of days later, with her middle names in tribute to her great-grandmother, the Queen and paternal late grandmother, Diana, Princess of Wales.
Charlotte was baptised at St Mary Magdalene Church in Sandringham, Norfolk in July 2015. Her parents' cousins, Laura Fellowes and Adam Middleton are among her godparents, as well as their friends Thomas van Straubenzee, James Meade and Sophie Carter.
The Princess made history at the age of two when her younger brother Louis was born in April 2018. Charlotte became the first female royal to benefit from the change of succession law, which states that girls will not be overtaken by any future younger brothers. This meant that she retained her place as fourth-in-line to the throne, while Louis is fifth.
Fans thought the royal looked like her father
Last year while she celebrated her birthday in lockdown, Charlotte joined her family in making batches of fresh pasta and filling at least a dozen bags to deliver along with food packages to elderly and vulnerable people living near Sandringham.
The Cambridges were doing their bit to help their local community amid the pandemic, and the series of photos taken by mum Kate showed Charlotte carrying food parcels and knocking on people's doors. The royals were taking part in Sandringham Estate's volunteering initiative, which saw staff prepare and deliver meals to isolated people living in the area.
Around 1,000 meals were made and delivered in the first week alone.
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