A slogan saying the high street bakery had been "providing sh*t to scum for over 70 years" appeared on an image on the first page of results shown to anyone searching the company's name.
Greggs' Wikipedia page was also targeted this afternoon with a crude message, quickly removed.
Greggs on its official Twitter feed said it was aware of the change and was speaking to Google about it.
Greggs said on Twitter to users who had spotted the logo: "All publicity is good publicity? That's what they say isn't it? *weeps alone in a corner*".
In another it said: "We've been working with Google all morning to try and fix it! Hopefully get it sorted soon!!"
Google was not immediately available for comment.
The logo was hosted by the website Uncyclopedia, which reveals it was uploaded by a user called Romartus this morning.
A Greggs spokeswoman said: "We are aware of the abusive logo and have made a formal complaint to Google and asked them to rectify this as a matter of urgency."