Sankt Lars kyrkoruin, Medieval church ruin in Sigtuna Municipality, Sweden

Sankt Lars kyrkoruin is a church ruin on Prästgatan in Sigtuna, of which only the collapsed west tower remains above ground. The tower is built of rough grey and brown stones in uneven courses, with a broken top and a small patch of red brick near the upper right. The wall is thicker and stepped at one corner, and lower rubble lies at its base.

The church was built in the 1100s during Sigtuna's Christianization and was enlarged eastward and northward in the late Middle Ages, with a porch on the south side. It remained in use until the Reformation; after repairs in 1586 the building served as a school. A fire in 1658 left it a ruin, and today only the collapsed west tower stands above ground.

Sankt Lars was regarded as Sigtuna's actual town church and its foremost one, close to the medieval market square. A modern historian has suggested it, not Sankt Per, served as the town's cathedral before the bishop's seat moved to Gamla Uppsala. A 17th-century historian compared its silhouette to Sankt Pers and Sankt Nicolai churches.

The ruin stands on the east side of Prästgatan, in the block between the medieval street grid and the museum area, so it can be seen on foot while walking the street. Only the broken stump of the west tower rises above ground; the rest of the church lies beneath the grass. The ground around the stones is uneven and there are no facilities at the site.

A 17th-century historian wrote that the church's silhouette was as striking as those of Sankt Pers kyrkoruin or Sankt Nicolai. Today only the ruined tower stands, with parts of the west tower still above ground. The bell tower on Klockbacken, west of the ruin and above Prästgatan, burned down on 30 June 2016 and has been rebuilt on the same spot.