In Acts, Matthias was chosen to replace Judas Iscariot among the apostles.
Acts describes the believers gathering in Jerusalem after Judas’s death. Peter says a replacement should be selected from men who had accompanied Jesus from his baptism through the ascension. Two candidates are proposed: Joseph called Barsabbas, also known as Justus, and Matthias.
The group prays and casts lots, and Matthias is counted with the eleven apostles. This restores the symbolic number twelve before the narrative moves into the events of Pentecost. Acts does not give Matthias a major speaking role afterward.
Matthias is not the same person as Matthew, the tax collector and apostle traditionally associated with the first Gospel. He is also distinct from Paul, whose conversion and missionary work become central later in Acts. The replacement episode is found specifically in Acts chapter 1.