Rayo had spurned an opportunity to seal their return to the top flight last week by losing 4-0 at Alcorcon but they seized on their second chance thanks to a volley from winger Alex Moreno in the 39th minute and held on to their lead with the help of a boisterous atmosphere.
Their promotion means there will be five teams from the Spanish capital playing in La Liga next season for the first time ever, joining Getafe, Leganes, Atletico Madrid and Real Madrid.
Rayo gained international attention by releasing an away strip in 2015 to donate to seven social causes including LGBT rights, and they also paid the rent of a local supporter who had been evicted from her home in the wake of the recent economic crisis.
Rayo top the Segunda Division standings on 76 points with one game remaining, one above Huesca who sealed their promotion to La Liga for the first time last Monday.
(Reporting by Richard Martin; Editing by Clare Fallon)
