Catriona Barr was wild swimming in the sea near Lerwick when she spotted the bull orca underneath her.
Her attention was drawn to the mammal after she noticed people on the shore were watching her.
Erik Isbister, who was watching from the shoreline, believed the orca may have mistaken Ms Barr for a seal.
The encounter happened at Da Sletts at Breiwick on Wednesday and was filmed from shore.
Ms Barr swims in the sea almost every day and said she was used to seeing people walking along the nearby shore.
She also said she was aware of the risks of swimming in the sea, but had never before come across orcas on her swims.
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