Isabel Flores de Oliva was born on April 20, 1586 in Lima,[3] the capital of the then Viceroyalty of Peru. She was the daughter of the Spaniard Gaspar Flores, a native of Spain, and María de Oliva. She was the fourth daughter of twelve siblings, of which only nine are known: Gaspar, Hernando, Bernandina, Francisco, Juana, Antonio, Andrés, Jacinta, and Francisco Matia.

At three months old, a maid claimed to have seen her face transform into a rose, and from then on she was called Rose by her mother.

On the day of her confirmation, in the town of Quives, Archbishop Toribio de Mogrovejo confirmed her with the name Rosa. Although it mortified her to be called that because they were making fun of her, since she was the only one with that name.

Due to her family's financial difficulties, she worked all day in the garden and embroidered for different families in the city to help support her family. Under these precarious conditions, she also saw another, more humiliating poverty around her: that of the Indians. Her governess, Mariana, who was Indian, helped her become aware of the humility of these individuals.

"If Christians are obliged to preach love everywhere, why did they come to America with wars, destruction and hatred?"

That thought tortured her, and she wondered in anguish: "Why must so many Indians suffer?" She found no answer until she discovered the redemptive value of suffering.

When she was admired for her beauty, Rosa cut her hair and threw pepper on her face, annoyed at having attracted suitors. She rejected all her suitors, despite the opposition of friends and family. Rosa spent several hours a day observing the Blessed Sacrament.

Rosa attracted the attention of the friars of the Dominican Order. She wanted to become a nun, but her father forbade her, so after a few years she joined the Third Order of Saint Dominic, imitating her admired Saint Catherine of Siena.