Her visit came the day before the Cardiff Half Marathon.
The retired middle-distance runner, who won gold in the 800m and 1500m at the 2004 Athens Olympics, joined runners, walkers, and volunteers for the weekly 5k event along the promenade.
In a social media post, organisers said Ms Holmes had supported the event by selecting Barry Island parkrun to receive one of her birthday donations earlier this year.
The post said: “A few months ago Kelly picked Barry Island to win one of her birthday donations which purchased much of the new kit you see – and today she came to see it in its glory.
“Thank you for your patience Kelly – taking lots of photos with our runners, walkers and volunteers.”
Ms Holmes said she thoroughly enjoyed her morning at the event.
In a post on social media, she said: “What a morning!
“I have to say everyone here were amazing. The volunteers are so warm and enthusiastic the Parkrunners/walkers lovely!
“Two lap course along the promenade with hills.
“I had a little chat at the start talking about why I love parkrun
“I ran with a lady named Karen who was on her first lap to keep her company before I finished my second.
“Had photos with nearly all the parkrunners/walkers, including lots of doggie pics.
“Loved the volunteers as always.
“I spoke with Jim Cox – 710 parkruns. My spot of the week!”
Ms Holmes also enjoyed coffee and cake at Marco’s Barry Island, and homemade ice cream at Zio’s Gelato.
Her visit is the latest in a series of special appearances at the event, which celebrated its 500th run in July with 510 participants.
Since it began on May 2, 2015, Barry Island parkrun has seen 14,825 people complete 72,781 parkruns, covering 363,905km, and recording 9,947 personal bests.
The event has welcomed other notable guests, including BBC Weather presenters Derek Brockway and Sue Charles, who acted as official starters at the 500th event.
