65-year-old fitness legend Denise Austin showed off her incredible physique while strutting the runway alongside her daughter at Miami Swim Week ... throwing on a bikini and absolutely crushing it!!

The workout video icon walked alongside 29-year-old Katie -- a Sports Illustrated Swimsuit rookie -- over the weekend ... making them the first mother and daughter to rock the runway together at the event.

Of course, Austin became a household name after making more than 50 fitness tapes over several decades ... and her daughter has followed in her footsteps with her own health-focused app.

Austin spoke about the experience ... saying it was "A ONCE IN A LIFETIME MOMENT!!"

"With my daughter @katieaustin walking in the @si_swimsuit runway show!! Me at 65 in a bikini ... with my little girl!!!!!! Pure joy!!"

Katie raved about the special opportunity ... adding, "Hugging my mom on the runway was such a cute moment. We passed each other and got to say I love you."

"I really cherished every moment of this past weekend and feel so grateful of the opportunities in my life!!!!"