Lukaku has been linked with a move away from the Italian champions following the departure of head coach Antonio Conte, as the Nerazzurri struggle financially due to the coronavirus pandemic.
The 28-year-old has reportedly caught the attention of his former club Chelsea and Premier League champion Manchester City.
Speaking ahead of Inter's friendly with Pergolettese, Marotta quashed talk of Lukaku leaving San Siro.
"From our side, we can yes absolutely, Lukaku is not for sale," Marotta said. "Lukaku is an important piece on the chess board available to [head coach} Simone Inzaghi."
Lukaku scored 24 goals from 36 league outings in 2020-2021, while he also provided 11 assists.
Only in 2016-2017 for Everton did the Belgian score more (25) in a single campaign in one of Europe's top five leagues.
Lukaku is expected to train for the first time under Inzaghi on Tuesday (AEST).