Sejerø is a Danish island in the Kattegat sea, north-west of Zealand. Sejerø with its 328 inhabitants is part of Kalundborg Municipality and covers an area of 12.36 square kilometres. The largest village on Sejerø is Sejerby, housing half of the island's population. South-east of Sejerø is the smaller island of Nekselø, also in the Bay of Sejerø.