Joining other royals on the balcony of the Imperial Palace in Tokyo, Masako looked the most at ease she has in years. The princess, who is married to Crown Prince Naruhito, gave a friendly wave to well-wishers as she marked her father-in-law's big day.
Masako was dressed elegantly in a burgundy outfit that she paired with a hat of the same colour, white gloves, an elaborate brooch and a strand of pearls.
Princess Masako has been recovering from adjustment disorder
Masako's attendance at the very public birthday celebrations is yet another indicator that she is gradually making a recovery.
The princess was first diagnosed with a stress-induced adjustment disorder, also known as situational depression, in 2004. Several reports claimed that the royal, who has a daughter Aiko with her husband, felt under pressure to produce a male heir and live a life in the royal spotlight.
The princess was on positive form as she celebrated her father-in-law's 82nd birthday
However in the past year Masako has been making every effort to fulfil public duties, while she carries on with her treatment. On the occasion of her 52nd birthday earlier this month, she reflected on her progress.
"Taking care of my health, I have tried to fulfil my public and private duties as much as possible this year," Masako wrote in a statement released by the Imperial Household Agency. "With the help of people around me, I would like to continue making efforts to recover."
In a separate statement her medical team added that Masako has been working hard to be physically fit and to increase her number of engagements.
Masako has made more and more public appearances this year
Last month the crown princess attended – for the first time in 12 years – a biannual garden party hosted by Emperor Akihito and Empress Michiko. In the presence of 2,000 guests, Masako mingled with attendees and looked comfortable in her surroundings.
The royal also made an official trip to Tonga in July to attend the coronation of King Tupou VI. The decision for Masako to travel was not made lightly, and it was only after careful consideration and in-depth talks with her doctors that she was given the go-ahead.