Fortnam-Tomlinson, 23, scored 10 goals for Stamford last season and provides a timely boost for the Gingerbreads ahead of their crunch clash with Marine at the weekend.
Manager Richard Thomas said: "Bradley fits what we need a goal scorer with experience."
The striker has previously played for Stafford Rangers, St Johnstone, Grays, Tonbridge Angels, St Neots, Ilkeston, Hereford and Bishop's Stortford.
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