The 20-year-old Scotland Under-21 international began his career at Reading but is yet to make his senior debut for the Royals.
Hyam had loan spells at Basingstoke and Dagenham & Redbridge last season, playing 16 times for the Daggers as they were relegated from League Two.
He has also previously played for non-league side Hemel Hempstead on loan.
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League Two club Portsmouth have signed defender Dominic Hyam on loan from Championship Reading until 8 January.