The coin in the fish's mouth is one of the miracles of Jesus, told in the Gospel of Matthew 17:24-27.
In the Gospel account, in Capernaum the collectors of the two-drachma temple tax ask Peter whether Jesus pays the tax, and he replies that he does. When Peter returns to where they are staying, Jesus speaks of the matter, asking his opinion: "From whom do the kings of the earth collect duty and taxes — from their own children or from others?" Peter answers, "from others," and Jesus replies: "Then the children are exempt. But so that we may not cause offense, go to the lake and throw out your line. Take the first fish you catch; open its mouth and you will find a four-drachma coin. Take it and give it to them for my tax and yours."
The story ends at this point, without stating that Peter caught the fish as Jesus predicted.
The four-drachma (or shekel) coin would be exactly enough to pay the temple tax (two-drachma coin) for two people. It is usually thought to be a Tyrian shekel.
The coin in the fish's mouth is generally seen as a symbolic act or sign, but there is little agreement concerning what it signifies.
The Bible does not specify the species of the fish caught by Peter, but Tilapia is sometimes referred to as "St. Peter's fish".