Cornelia Maria (Cocky) van Engelsdorp-Gastelaars (born 24 February 1938, Rotterdam) is a Dutch former swimmer who twice improved the world record in the women's 100 metre freestyle in 1956.

On March 3, 1956, in Amsterdam she broke the 20-year-old record of 1:04.6 by Willy den Ouden, who like Gastelaars, was born and raised in Rotterdam.

Only later it turned out that her time of 1:04.2 was an improvement on Dawn Fraser race in 1:04.5 in Sydney the previous week.

On April 14 Gastelaars lowered the record to 1:04.0 in her home base swimming pool in Schiedam.

She did compete in the 1960 Summer Olympics, but was past her prime at that point and finished seventh in 100m freestyle.