The 37-year-old Spaniard left Formula One at the end of last season but remains close to the team as an ambassador and also as a driver competing in the Indianapolis 500 with a McLaren entry in May.
McLaren have an all-new lineup this year in Spaniard Carlos Sainz and British rookie Lando Norris.
McLaren said Alonso will carry out tyre testing duties for Pirelli at Sakhir on Tuesday and Wednesday next week in a second car provided by the team in addition to the MCL34.
The Bahrain in-season test will also feature Michael Schumacher's 19-year-old son Mick, driving for both Ferrari and Alfa Romeo.
British racer Dan Ticktum will make his test debut for Red Bull, who also have Max Verstappen at the test.
(Reporting by Alan Baldwin, editing by Pritha Sarkar)
