Poli, 65, was named to the position by Francis in late March and Vatican envoy Emil Paul Tscherrig welcomed him in the name of the pope in the ceremony at the capital's metropolitan cathedral, saying the pontiff "accompanies us from Rome."
Argentine President Cristina Fernandez, who was traveling in Peru and Venezuela, did not attend the ceremony but several of her Cabinet ministers did.
"The arrival of the replacement for Bergoglio generates enthusiasm, expectations and hope, and we wish him the best in his pastoral activities in Buenos Aires," said Julian Dominguez, the head of the Chamber of Deputies.
Poli is a priest very much in Francis' vein, focused on pastoral work. He has been the bishop in Santa Rosa in the rural La Pampa province, and from 2002-2008 he served as one of then-Cardinal Bergoglio's auxiliary bishops in Buenos Aires, but he was not among the church officials rumored to be top candidates for the post.
He has a doctorate in theology from the Catholic University of Argentina, but before he was ordained as a priest in 1978, he earned a degree in social work from the public University of Buenos Aires, known as UBA. That apparently impressed Bergoglio, who talked about the benefits of such training in a 2012 book, "On Heaven and Earth."