Labour member Freddie Bailey, who represents the ward containing the Masjid e Salaam mosque in Fulwood, slammed the vandalism as an "act of hatred" and appealed for people to identify the culprits.
Two gateposts at the mosque in Watling Street Road had offensive slogans daubed on them in marker pen.
Mukhtar Master, Muslim representative on the Preston City Council Faith Covenant, described the attack as "an isolated racist incident."
He said: "The Salaam Mosque, just as all other mosques in Preston, work tirelessly in trying to develop and maintain good relationships with all sections of our wonderful community.
"This is clearly an isolated racist incident and as such we condemn it, just as all others would condemn it also.
"However we will not let this discourage us from continuing to reach out in peace and harmony with the rest of our community."
Coun Bailey tweeted: "Unfortunately last night someone graffitied the mosque in the area I represent in Preston.
"I am absolutely disgusted. There is no excuse for this act of hatred.
"Anyone with information regarding this please message me."
Another message on Twitter, from Talha Musa, showing photographs of the graffiti, said: "So this just happened to a mosque in Preston.
"Vile, disgusting behaviour."