"Wishing a very happy birthday to The Duchess of Cambridge today," they wrote.
Their comment included a birthday cake and red heart emoji.
While some Instagram users just appreciated the message dedicated to Kate, others expressed their disappointment at the Duke and Duchess of Sussex.
Kate was seen arriving at Kensington Palace looking stoney-faced in the wake of the announcement by the Duke and Duchess of Sussex.
Royal author Phil Dampier told Fabulous Digital that Kate's special day was undoubtedly ruined by Meghan and Harry's news.
“Apart from anything else Harry and Meghan’s announcement has overshadowed Kate’s 38th birthday.
“It’s just another example how far the so-called Fab Four have drifted apart and I doubt Harry and Meghan will be at any celebrations for Kate.
“It’s a sad day for the monarchy that these two brothers and their wives, who had the potential to be such a positive force are now in this position," he told the publication.