Renault had already appointed the former Williams racer in February and McLaren said an agreement had been reached for Sirotkin to fill in for them too should Carlos Sainz or Lando Norris be unavailable.
"This is an expedient solution for McLaren in 2019," said McLaren team principal Andreas Seidl in a statement.
"Sergey is a known quantity, has the necessary superlicence points and is formally aligned to our engine provider, Renault, so he is the obvious candidate."
Sirotkin, 23, will also continue with SMP Racing in the World Endurance Championship. He will attend every grand prix for the rest of the year.
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(Reporting by Alan Baldwin, editing by Toby Davis)
