Pope Francis delivered a message via video link ahead of the illumination of the star which is made of steel and glass and sits atop the temple's 138 meter high pinnacle.
Cardinal Joan Josep Omella I Omella blessed the star and the tower before the lights were turned on in stages. The 800 windows were illuminated first, followed by the star.
"It's a special moment because the tower after such a long time is finally finished and we have the star," said Dutch tourist Leonie Wunker.
Work on Gaudi's Sagrada Familia began in 1882.