Trefaldighetskyrkan

The church is an octagonal, centrally planned building beneath a dome, a form inspired by Roman Baroque models—possibly Santa Maria dei Miracoli or the Pantheon in Rome. It was designed by Nicodemus Tessin the Younger, who also created Fredrikskyrkan nearby and Stockholm Palace, drawing on French and Italian Baroque architecture. Some accounts also point to Palladio's Villa Rotonda as a possible influence.

The congregation that built the church was formed by German settlers who came to Karlskrona in its early years after the town's founding in 1680, initially worshipping in the temporary wooden Hedvig Eleonora church before gaining permission to build their own place of worship. The foundation stone was laid in 1697, and although the church was consecrated in 1709, building work continued for a further forty years.

A devastating town fire in 1790 gutted the church, leaving only its blackened outer walls standing. Two brass chandeliers from 1722 and a memorial tablet from 1789 bearing a relief of the church were rescued and still hang inside today. Reconstruction began the following year to designs by Olof Tempelman, who simplified Tessin's original scheme and gave the dome a lower, stepped profile in the neoclassical taste of the period, again echoing the Pantheon. The dome's painted decoration, completed by the sculptor Johan Törnström and his son Emmanuel, uses trompe l'oeil effects to make the space appear taller than it is.//At its reopening in 1802 the church was briefly named after Queen Fredrica Dorothea Wilhelmina, only taking its present name after the King's deposition and the royal family's exile.

Inside, the chancel is dominated by a neoclassical altar composition with columns and, above it, a large gilded cross with a shroud and crown of thorns, originally made for Fredrikskyrkan to a design attributed to Johan Adolf Hawerman and carved by Carl Gustaf Glader, a figurehead carver at the Karlskrona shipyard; it was moved into Trefaldighetskyrkan by 1949. Johan Törnström, who is buried in the churchyard, is also credited with the unusual empire-style pulpit and other woodwork. The red limestone font, from northern Öland, is older than the current building and was given to the church in 1658, having previously belonged to Hedvig Eleonora's old town church.

The present organ was built in 1827 by Pehr Zacharias Strand of Stockholm, with a case designed by the architect Carl-Gustaf Blom-Carlsson; it replaced an earlier instrument lost in the 1790 fire. It is said to be among the best-preserved of Strand's surviving organs. A smaller continental organ, built in Germany in 1993 and originally installed in Fredrikskyrkan, was moved to the chancel here in 2015.

A curious detail rewards visitors who climb to the gallery: seen from above, the arrangement of the pews is said to mirror a historic map of the town's streets and squares, divided into four quarters. The four corners of the church also contain distinct niches reserved for the Wachtmeister family, positioned so that its members sat higher than the priest below.