Sidney Harry Jeffryes (20 July 1884 – 16 October 1942) was an early Australian wireless telegraphy operator. Trained by Australasian Wireless Co., Ltd., he was initially employed on coastal shipping and established at least one record for distance transmission. But he is best known for his service as the wireless officer at Cape Denison during the second year (February 1913 − December 1913) of the Australasian Antarctic Expedition under the command of explorer Douglas Mawson. His service ended in September 1913, two months prior to the relief of the shore party, when Jeffryes developed symptoms of paranoia and had to be relieved of his duties.