The advert has provoked a debate about whether it patronises or empowers women in a country long dominated by a macho culture.
"Thank you Irene Montero" began trending sardonically on Spanish Twitter as users pointed out that women of all shapes and sizes visited Spain's beaches every summer.
"You are up there with Nelson Mandela," tweeted Santiago Armesilla, a Left-wing writer, ironically thanking the minister for repealing the laws that kept obese women off Spain's beaches.
Spain's equality ministry has pursued a raft of progressive policies, including introducing a Europe-first right to paid menstrual leave.
Jorge Buxadé, the leader of Vox's MEPs in the European Parliament, described the campaign as an affront to working people who are struggling to pay their bills during the cost of living crisis. "They are sick of your neuroses," Buxadé tweeted to Montero.
Cayo Lara, the former leader of the United Left group, now a part of Unidas Podemos alongside Montero, said the campaign was "creating a problem where none exists".
Ángela Rodríguez Pam, a junior equality minister, sought to justify the campaign, saying it wasn't about giving women permission to go to the beach but boosting the confidence of those who are nervous about their shape.
"Of course we go [to the beach], but in the knowledge that we receive hatred for showing a non-standard body," Rodríguez Pam said.
'Free of stereotypes and violence'
The Institute of Women, an agency of the Equality Ministry, said the poster release was just the first part of a campaign which will focus on how women are portrayed in advertising.
"It is essential to recognise the existence of body diversity and to reclaim it, free of stereotypes and violence," said Toni Morillas, director of the Institute of Women.
According to the Fita Foundation, which helps people with mental health issues, around 400,000 people in Spain are obese.
Between 4 and 6 per cent of adolescents between the ages of 12 and 21 suffer from anorexia or other eating disorders, 90 per cent of whom are girls.
While Spanish social media had a meltdown over the beach poster, there was some support from abroad for the message of inclusivity regarding women's bodies.