The disciple who replaced Judas Iscariot was Matthias. According to the Bible, after the death of Jesus, the remaining 11 apostles chose Matthias to replace Judas, who had betrayed Jesus and later died. This was done to ensure that there were always 12 apostles, as Jesus had originally chosen 12 to be his closest followers. The selection of Matthias was done through a process of prayer and casting of lots, and he became the 12th apostle.