Petronella Angela "Petra" Hillenius (7 February 1968 in Woerden^[1] - 27 March 2020) was a Dutch breaststroke swimmer, who competed at the 1984 Olympics in Los Angeles.^[2] She finished 19th in the 100 metre breaststroke and 18th in the 200 metre breaststroke.

In 2014, she became a board member of the Royal Dutch Swimming Federation.^[3] She resigned the position when she was diagnosed with ALS four years later. In March 2020 she died due to ALS.^[4]

She was a teacher for 24 years at the Hospitality Business School of the Saxion University of Applied Sciences.