Shaped like a three-aisle basilica, St. John's Church is Finland's largest stone church - it originally had 3000 seats. Several rows of benches have been removed in recent years to make the church space more flexible. Today the number of seats is just over 2200. The church has been filled to capacity on two occasions: the inauguration and the funeral of Zacharias Topelius.

The length of the church is 59,38mand the nave is internally 42.75m long. The towers are 74,22m high. St. John's Church is one of two churches in Finland with double towers. The church bells that produce the first three notes (ciss, e and fiss) of the hymn Te Deum were installed in 1932.

The colored window glass, as well as the fixtures and altar decorations, were supplied by Josef Leja in Stockholm. On the four brass wall lamps nearest the altar are the symbols of the four evangelists. Next to the main entrance of the church is a statue of John the Baptist. It was made in 2003 by sculptor Kari Juva.