The 25-year-old is reunited with manager Martin Ling, whom he briefly played under at Leyton Orient before the O's parted company with Ling's services - and that of former assistant and current Saddlers boss Dean Smith - in January 2009.
Jarvis joined Walsall last summer and was involved heavily over August and September, scoring three goals in his first seven games.
However, first-team opportunities have proved hard to come by at the Banks's Stadium as the campaign has worn on, with just 13 appearances without a single goal between October and February.
Jarvis goes straight into contention to make his debut in Saturday's trip to Crawley.
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