His meeting with Ramakrishna Paramahamsa in November 1881 proved to be a turning point in his life.About this meeting, Narendranath said, "He [Ramakrishna] looked just like an ordinary man, with nothing remarkable about him. He used the most simple language and I thought 'Can this man be a great teacher?'– I crept near to him and asked him the question which I had been asking others all my life: 'Do you believe in God, Sir?' 'Yes,' he replied. 'Can you prove it, Sir?' 'Yes.' 'How?' 'Because I see Him just as I see you here, only in a much intenser sense.' That impressed me at once. I began to go to that man, day after day, and I actually saw that religion could be given. One touch, one glance, can change a whole life."
Even though Narendra did not accept Ramakrishna as his guru initially and revolted against his ideas, he was attracted by his personality and visited him frequently. He initially looked upon on Ramakrishna's ecstasies and visions as, "mere figments of imagination", "mere hallucinations". As a member of Brahmo samaj, he revolted against idol worship and polytheism, and Ramakrishna's worship of Kali. He even rejected the Advaitist Vedantism of identity with absolute as blasphemy and madness, and often made fun of the concept
Though Narendra could not accept Ramakrishna and his visions, he could not neglect him either. It had always been in Narendra's nature to test something thoroughly before he would accept it. He tested Ramakrishna, who never asked Narendra to abandon reason, and faced all of Narendra's arguments and examinations with patience—"Try to see the truth from all angles" was his reply. During the course of five years of his training under Ramakrishna, Narendra was transformed from a restless, puzzled, impatient youth to a mature man who was ready to renounce everything for the sake of God-realization. In time, Narendra accepted Ramakrishna, and when he accepted, his acceptance was whole-hearted.
In 1885 Ramakrishna suffered from throat cancer and he was shifted to Calcutta and later to Cossipore. Vivekananda and his brother disciples took care of Ramakrishna during this final days. His spiritual education under Ramakrishna continued here. At Cossipore, Vivekananda reportedly experienced Nirvikalpa Samadhi.During the last days of Ramakrishna, Vivekananda and some of the other disciples received the ochre monastic robes from Ramakrishna, which formed the first monastic order of Ramakrishna. Vivekananda was taught that service to men was the most effective worship of God. It is reported that when Vivekananda, doubted Ramakrishna's claim of avatara, Ramakrishna reportedly said, "He who was Rama, He who was Krishna, He himself is now Ramakrishna in this body." During his final days, Ramakrishna asked Vivekananda to take care of other monastic disciples and in turn asked them to look upon Vivekananda as their leader.Ramakrishna's condition worsened gradually and he expired in the early morning hours of August 16, 1886 at the Cossipore garden house. According to his disciples, this was Mahasamadhi.