Kärnbo ödekyrka, Church ruin in Mariefred, Strängnäs Municipality, Sweden

Kärnbo church ruin consists of grey stone walls forming a Romanesque apse church, built largely in the later 12th century. In the late Middle Ages the church was extended westwards and a porch was added. The ruin stands on a hill north of Gripsholm royal demesne, west of Mariefred's old town centre.

The building was constructed in the late 12th century and expanded over time. After Mariefred was granted town privileges in 1605, the structure gradually ceased to serve as a church.

The runestone Sö 177 is set into one of the church walls at the entrance. It was likely raised nearby in the late Viking Age and walled in when the church was built in the late 12th century. The church served as the burial place of regent Sten Sture the Elder for part of the 16th century; his coffin was moved here from Mariefred's monastery church and later, in 1576, to Strängnäs Cathedral.

The ruin stands on a hill west of Mariefred's old town centre, north of Gripsholm Royal Estate, and is reached on foot. The terrain is uneven and the site is not accessible for people with mobility issues. There are no facilities or set opening hours.

The runestone Sö 177 is set into one of the church walls at the entrance. It was probably raised nearby in the late Viking Age and was walled in when the church was built in the late 12th century.