Ugwu's initial month-long spell at Home Park has been hampered by an ankle injury.
The teenage striker's deal expired following Saturday's 1-0 home defeat to Oxford but he will now remain with the Pilgrims until March 23.
The game against Oxford was the 19-year-old's full debut for Plymouth.
A training-ground ankle injury had kept him out until the previous weekend, when he had a brief taste of action as a second-half substitute in the 2-1 victory at Aldershot.
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