The British artist writes on social media that she has been informed about problems "that are not compatible with the groups in society that I support".
Melanie C has not specified why she chose to cancel the concert, but the stance has been praised by LGBTQ people.
Polish authorities and the television channel TVP, which organized the concert, have been widely criticized for their stance on the rights of LGBTQ people.
In Poland, same-sex marriage is not legal, and same-sex couples do not have the right to adopt children.
TVP has previously been described by Reporters Without Borders as "an instrument of state propaganda".
The TV channel states in a statement that Melanie C canceled the concert after "pressure from online comments".