A crew from Worthing attended Edgehill Close at 12pm, where they found the gull entangled in netting on the roof of a house.
Despite their quick intervention, it was too late to save it, and it died shortly afterwards.
It it thought the seagull had been trapped for several days, but no-one had noticed it.
Firefighter Andy Gilbert said reports of birds getting trapped in such netting were not uncommon, and he knew of three similar incidents in the past three weeks.
He said: “It is torture for the bird. It would have been killed slowly.
“This one was a yearling, which had returned to the nest.
“I would like to show those who put up the netting what they have done. If you don’t like gulls nesting, don’t live by the sea. We invaded their space, not the other way.”
A spokeswoman for the RSPCA said it was an offence to cause unnecessary suffering to animals and nets should be maintained in order to prevent such incidents from occurring.