The insults appear on a Facebook page called "Waitrose Isn't a Supermarket, It's a State of Mind", which asks staff to list what annoys them most.
Dozens of employees posted comments on the group's wall.
One wrote: "There's a gentleman who comes in every Monday who has probably never showered and stinks of p**s.
"This guy smells so f***ing bad that I could barely open my mouth to breathe or speak to him. People like that shud [sic] be shot!"
Another wrote: "Pikey skanks wait till the last minute, gathered around the reduced-stuff-bin, or the cake shelves etc, to get the cheapest possible stuff."
A Waitrose spokesperson said: "This is completely unacceptable behaviour. It goes against our codes of conduct which make it very clear that partners who post this type of material are in breach of their terms of employment.
"The nature of the internet makes it difficult to get content removed once posted, however we do have teams that monitor website activity and will be conducting an immediate investigation."
The news comes just days after Tesco launched a similar investigation into its staff following the revelation that some employees had posted abusive comments about customers on a Facebook group called "Tesco employees could rule the world".