Asda staff held a protest outside the company’s Bloxwich store where they held signs and blew horns, supported by representatives from the GMB Union.
The contract would see staff lose holidays and face stricter working conditions, according to union leaders, who said staff must sign the contract by November or risk losing their job.
Asda, which is owned by the US retailer Walmart, said the "overwhelming majority" of staff at Bloxwich and across the country had already signed the contract.
Elaine King, from the GMB, said: “Asda announced in March they were putting everyone onto one contract.
“On August 4, they dismissed everyone’s terms and conditions and told them they have got 12 weeks notice to sign this new contract.
“If they don't sign by November 2, at 23.59, they will be wiped off the system and lose their job completely.
“These people will stand to lose their flexibility. Asda are saying the can give them four-week notice and can move them to work in the store for any hours.
“They have shortened the night window so the night rate is not paid for a longer period.
"People who may have been here through the years and been loyal, and didn’t have to work bank holidays, are being told they will have to.
“All their paid breaks are being taken off them. It is ludicrous to be honest.”
Staff held signs saying “we demand a better deal” and “respect your workers” during what was one of 12 protests across the country.
Karen Fisher, aged 53 and from Bloxwich, has worked for Asda for 18 years.
She said: “There has been no loyalty to any colleagues.
“You could be someone who has just walked off the street and done no loyalty at all, and you are being treated the same as colleagues who have just come in.”
An Asda spokesman said: “The retail sector is immensely competitive and it is important that we are able to serve our customers in the best way to meet their needs.
"This contract is about increasing the take-home pay of more than 100,000 retail colleagues, through an investment of more than £80 million, and ensuring that everyone doing the same job is on the same terms and conditions.
“The overwhelming majority of our colleagues from across all our stores, including Bloxwich, have signed onto the new contracts and while we appreciate that some of our colleagues find the changes more unsettling, we do not want any of them to leave.
"We understand colleagues have commitments outside of work and will not be asking them to constantly move the time they work, their days or departments.”