It was pointed out that a card for a wife was on sale at £2 - while a similar one for husbands was priced at £2.50.
Sainsbury's has promised to sell cards at the same price after eagle-eyed shopper Julie Marlow tweeted an image of the two cards side-by-side yesterday morning, and asked Sainsbury's: "Why do I have to pay 50p more for a card for my husband than he has to spend for virtually the same card?"
The supermarket said it would make sure both the cards were priced at £2 from Thursday.
The two cards have the same white background, both feature images of felt dogs and a felt heart, and both are part of the Sainsbury's Home range, but the card titled "For My Wife" was priced at £2, and the card titled "For My Husband" was 50p more expensive.
The Sainsbury's Twitter account later replied to Julie saying it "didn't seem right."
A Sainsbury's spokesperson said: "We appreciate this being brought to our attention. We've looked into it and you’re right, we made a mistake. Both designs are now available in store for £2."