Rosenborg got off to an impressive start with Magnus Blakstad firing the visitors into the lead after 25 minutes.
Inter responded well with Francesco Forte equalising just eight minutes later to take the sides into the break at 1-1.
The Italians looked the better side in the opening exchanges of the second half and they took the lead on 70 minutes through 16-year old Thomas Pedrabissi who slotted home from the edge of the box.
Forte grabbed his second goal of the game five minutes from time to seal the three points for the NextGen Series champions.
Inter's next group game is against Liverpool in January at St Helens' Langtree Park with the outcome determining whether the Reds will progress to the last 16 as one of the four best third place finishers.
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