who wrote the song joy to the world

The answer depends on which “Joy to the World” you mean:
- The famous Christmas hymn “Joy to the World” (often sung in churches) was written by Isaac Watts in 1719.
- The rock song “Joy to the World” that begins with “Jeremiah was a bullfrog” and was made famous by Three Dog Night was written by Hoyt Axton around 1970.