At the conclusion of Jesus’ final Passover in the upper room, Matthew and Mark’s gospels indicate that a hymn was sung (Mt. 26:30; Mk. 14:26). Is it possible to know what hymn they sang?

The Bible does not say; however, the hymn normally sung at the Passover was the last part of the “Hallel” (Ps.114-118 or 115-118). During this song, the leader would sing the lines and his followers would respond with “Hallelujah!”