The Belgium international completed his protracted move from Manchester United in a reported €80million (£73m) deal, ending weeks of speculation.
He did not feature at all for United during its pre-season campaign and was left out of Inter's squad for Sunday's friendly with Valencia.
However, he was part of a much-changed side that took to the field for a low-intensity encounter against Serie D minnow Bergamo.
Handed the number nine jersey, previously warn by Mauro Icardi, Lukaku made a good first impression as he netted half of Inter's goals in an 8-0 thrashing.
Matias Vecino and Sebastiano Esposito, two other players in need of minutes, scored braces to complete the rout.
Inter begins its Serie A campaign with a home match against Lecce on 26 August.