The 18-year-old, who had yet to make her professional debut with Sopela, was training on Thursday in Villares de la Reina when she was hit by a truck driver. Emergency services confirmed her death at the scene.
"We deeply regret the death of Estela Dominguez. Our most sincere condolences to her family, friends and teammates. All Spanish cycling is in mourning," the RFEC said in a statement.
"Please, always respect the cyclist on the road. Every bike carries a life."
She was the daughter of Juan Carlos Dominguez, who raced in the 1990s and 2000s, and also won a stage on the Giro d'Italia.
Tributes and condolences poured in after her death was announced, led by former professional cyclist Alejandro Valverde.
"There are no words for something like this. All my love to the family and friends of Estela Dominguez and especially her father, with whom I shared a squad in my early years," Valverde said.
Movistar team's Enric Mas said: "A big hug to the family of Estela Dominguez."
