Samuel, 22, had 15 goals in 72 appearances over the last two seasons with Fluminense, across all competitions for the club based in Rio de Janeiro.
Terms of the loan agreement were not disclosed.
"We are pleased to add Samuel to our roster," Galaxy General Manager and Coach Bruce Arena said in a statement.
"Samuel is a young, talented forward with both size and skill," Arena said. "We are confident this addition will help boost our attack in 2014."
With the Galaxy, the Brazilian will join two of the best forwards in