The 20-year-old, who signed his first professional deal with the Seagulls in 2013, is yet to play in a senior first-team match for the Championship side.
After previous loan spells at Braintree and Dartford, he made one start in League Two for Crawley last season.
"Tom brings to us a genuine left-footed option at centre back," Cambridge boss Shaun Derry told the club website.
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League Two club Cambridge United have signed Brighton defender Tom Dallison on a six-month loan deal.