St Ib's Church (Sankt Ibs Kirke) is a gem of Danish Romanesque architecture, tucked into the northern end of the old town a short walk from the cathedral. The church is small — it seats perhaps sixty people — and was built in the 12th century for the community that lived along the fjord. Unlike the cathedral, it was never substantially expanded, so it gives a good sense of what a Danish medieval parish church actually looked like.
The interior is whitewashed and spare, lit by small round-headed windows. There are medieval frescoes, though more fragmentary than in some Danish village churches. The churchyard is quiet and old. The church is easy to overlook on a quick visit to Roskilde, but if you walk down to the waterfront and the Viking Ship Museum it's almost directly on your route and worth the five-minute detour.