The church's oldest part is its narrow west tower, thought to survive from an earlier Romanesque building of the 1200s and notably out of proportion with the later nave. The rectangular nave, chancel and sacristy were added around 1330, along with a small prayer chamber in the south-west corner. The limewashed walls feature corner quoins and pointed windows with carved surrounds, while the tower is topped by an octagonal spire.
The church's most celebrated feature is its three Gothic portals, two on the south side and one on the north, each carrying richly carved capital friezes that together depict scenes from the life of Christ. This continuous sculpted narrative is considered unique among Gotland's medieval churches, and the work is thought to originate from the workshop of an anonymous stonemason sometimes referred to as 'Egypticus', active in the first half of the fourteenth century.
Inside, the nave and chancel are covered by cross vaults, and restoration work carried out in the early 1970s uncovered medieval wall paintings dating from the 1300s. The font is contemporary with the tower, from the 1200s, while the colourful pulpit, built in simple square panels showing the four evangelists, dates from the mid-1500s and is among the oldest in Sweden.
A small gallery at the west end, decorated with yellow and red scrollwork and said to date from 1682, was originally used as extra seating for visitors before an organ was installed there in 1865; a later mechanical organ was built in the 1970s. Other furnishings include a wooden altarpiece from 1675 and a memorial tablet from 1658 commemorating a local figure.
The churchyard is enclosed by a dry-stone wall built in the early 1800s, following a royal decree of the 1770s requiring such walls to keep livestock out of burial grounds. These walls are now valued as part of Sweden's shared ecclesiastical heritage, and the Martebo wall was restored in 2012.