Matthias (Hebrew transliteration: Mattityahu; Koine Greek: Μαθθίας; died c. 80 AD) was, according to the Acts of the Apostles, the apostle chosen by the believers to replace Judas Iscariot following Judas' betrayal of Jesus and his subsequent suicide. His calling as an apostle is unique, in that his appointment was not made personally by Jesus, who had already ascended into heaven, and it was also made before the descent of the Holy Spirit upon the early Church. The Biblical account in Acts 1:23-26 is as follows:

"So they proposed two, Joseph called Barsabbas, who was also called Justus, and Matthias. Then they prayed and said: '[...] designate which one of these two men you have chosen to take the place of this ministry and apostleship, from which Judas deviated to go to his own place.' So they cast lots over them, and the lot fell to Matthias, and he was counted along with the 11 apostles."

More Info: en.wikipedia.org