The present church was built in 1855–56, replacing a medieval church that had stood a little to the south. Builder Peter Isberg worked from designs by Johan Adolf Hawerman, who originally intended to retain the west tower of the older medieval church, a common practice when Öland's churches were rebuilt. However, Hawerman judged the old tower too fragile to incorporate, so an entirely new church was raised instead, and it was consecrated in July 1856.
From the outside, Kastlösa kyrka resembles many other Swedish churches of the 1800s: a plain hall church with a west tower topped by a lantern and a cross-crowned spire. Inside, though, the character changes markedly. A major renovation in the 1950s transformed the original hall-shaped interior into a three-aisled space with limestone columns and a flat, beamed ceiling, giving the church an unexpectedly weighty, architectural presence for its size.
The most memorable feature for visitors is likely to be the large fresco covering the east wall, where three windows were once located. Painted by Waldemar Lorentzon with assistance from artist Hans Fagerström and stuccoist Stig Kling, it depicts Christ standing on a rainbow above the flowering meadows of the Öland alvar, surrounded by people of many nations joining hands. The fresco also shows darker themes—Adam and Eve's fall and the destruction of Sodom—set against the hope represented by the nativity in Bethlehem and the cross rising above the whole composition.
Among the church's furnishings is a font with a drainage hole and a base of Gotland sandstone dating from the 1200s, one of the oldest surviving objects in the building. A crucifix from 1748, originally part of an altarpiece by Anders Dahlström the Elder, also survives, along with a small bell (primklocka) thought to date from the 1300s or 1400s. The stone altar, altar rail, pulpit, pews and gallery all date from the 1953–54 renovation, the pulpit notably carved from limestone with relief images of the evangelist symbols.
The church has had a succession of organs since 1858, rebuilt and altered several times through the 20th century, reflecting changing musical tastes and technical fashions rather than any single defining instrument.