Jan Sergiusz Gajek (Belarusian: Сяргей Гаек, romanized: Siarhiej Hajek; born Jan Gajek on 2 February 1949) is the Apostolic Administrator of the Belarusian Greek Catholic Church which is a sui iuris (independent) Eastern Catholic Church on the territory of Belarus.

He was born into a Polish Roman Catholic family in Łyszkowice, central Poland, and graduated from a local school and from a gymnasium.

He had contacts with the Belarusian cultural and religious centre in London, including bishop Ceslaus Sipovich and Robert Tamushanski.

The following two years Gajek studied at the Theology Faculty of the Catholic University of Lublin.

In 1994, Pope John Paul II appointed Gajek Apostolic Visitor for Greek Catholics in Belarus.