A crane on Friday placed the upper arm of a cross atop the Tower of Jesus Christ, the church's soaring central piece, which now stands 172.5 metres above the city, the church said.
While the unfinished monument is already the world's tallest church, with Friday's addition, the Sagrada Familia inched closer to being done.
The first stone of the Sagrada Familia was placed in 1882, but Gaudí never expected it to be completed in his lifetime.
Only one of its multiple towers was finished when he died at the age of 73 in 1926, after being hit by a tram.
In recent decades, work has sped up as the basilica became a major international tourist attraction with people around the world visiting Barcelona to see the monument, enthralled by Gaudí's radical aesthetic that combines Catholic symbolism and organic forms.
The inside of the Tower of Jesus Christ is still being worked on and its exterior is flanked by construction cranes and scaffolding.
But topping the central tower, which soars above the transept, has been a priority ahead of celebrations this June that will mark the centenary of Gaudí's death.
The scaffolding surrounding the central tower is expected to be removed by June, in time for the inauguration of the Tower of Jesus Christ, the church said.
As Gaudí had planned, the cross has four arms so its shape can be recognised from any direction, said Sagrada Familia's rector, the Reverend Josep Turull.
If Barcelona's city government allows it, the original plan also includes a light beam shining from each of the cross's arms, symbolising the church's role as a spiritual lighthouse, he added.
Millions of tourists visit the Sagrada Familia every year, and entrance fees largely fund the ongoing construction.
This year, the Sagrada Familia will hold several events to celebrate the Catalan Modernist's legacy, which includes other stunning buildings in Barcelona and elsewhere in Spain.
The Sagrada Familia became the world's largest church last October, when it rose above the spire of Germany's Ulmer Münster, a Gothic Lutheran church built over 347 years starting in 1543.
That church tops out at 161.53 metres.
A prayer verse from the Gloria that Catholics recite at mass is installed at the base of the cross installed Friday afternoon at Sagrada Familia, the church's rector said.
It reads: "You alone are the Holy One, you alone are the Lord, you alone are the Most High."
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