Hagby is one of only eight surviving round churches in Sweden, a form otherwise most closely associated with the Danish island of Bornholm. Its medieval exterior, including the apse and the tall upper-storey walls, survives in unusually complete condition, making it one of the finest examples of the type still standing.
The church was built in stone during the 1200s, replacing, some accounts suggest, an earlier wooden chapel dedicated to Saint Sigfrid a few kilometres away, though the connection between the two is uncertain given the chapel's lack of firm dating. Traditionally the church has been described as a defensive stronghold as well as a place of worship, its upper walls pierced by seventeen openings once interpreted as arrow- and missile-slits. More recent research has questioned how prominent this military role really was, suggesting the upper floor may instead have served as a well-lit ceremonial space. A central tower once stood inside the building, removed, it is thought, during the 1660s.
Soon after completion the church was damaged by fire, which scorched the walls up to the barrel vault; a south portal, side-altar niches and a cupboard recess were broken open in the walls as a result. The present timber barrel vault, covered with wooden shingles, dates from renovation work in the 1830s, when the windows were also enlarged and new ones added.
Inside, fragments of medieval wall paintings survive on the chancel wall and beneath where the organ gallery once stood, along with consecration crosses. A medieval font and a wooden triumphal crucifix, attributed to a regional workshop of around 1500, remain notable survivals from the church's earliest centuries. An altarpiece from the 1760s partly survives alongside some of its original figures, though a new altarpiece was introduced in 1923.
A thorough restoration between 2010 and 2016 aimed to return the interior to something closer to its original, austere character while using contemporary materials and design. Fittings including the organ gallery, pews and pulpit were removed, the wooden floor was replaced with stone, and lowering the floor level revealed the original bench seating built along the curved wall, which is now back in use. The church reopened in May 2016 following this work.