Norris, who won this year's European Formula 3 Championship, drove a current F1 car for the first time when he tested for McLaren in the post-Hungarian Grand Prix test in August.
The Brit's 2018 duties will see him attend races, test on-track, and drive the simulator.
McLaren said he is “currently assessing the opportunities and budget required for a full single-seater racing campaign” next year in addition to the F1 role.
Norris said, “The last 12 months have been an incredible journey for me – I've enjoyed every single minute – and this is just an amazing way to bring the year to an end.
“I'm really looking forward to helping the team, starting next week with the tyre test at Interlagos and I will work harder than ever to learn and develop across the next year.
“I have two excellent teachers in Fernando (Alonso) and Stoffel (Vandoorne), and I will do my best to help with the team's efforts to get back to the front.
“I'm still working hard to finalise my own racing programme for 2018 – we're close on a number of different fronts, but not quite there yet.”
He added his own racing programme “will be either Super Formula or Formula 2” and that he hopes to be able to announce something “shortly”.
McLaren racing director Eric Boullier said, “Lando is somebody who arrives with a very high level of ability, ambition and application.
“It is our job to develop and fine-tune his focus as we increase his understanding of how a Formula 1 team operates and how a modern grand prix car works.
“There will be plenty of opportunity to observe, learn and participate throughout 2018, and we want to make sure Lando is fully prepared, if needed, to step into Fernando or Stoffel's shoes at a moment's notice.”
McLaren executive director Zak Brown added, “He is undoubtedly carving himself a reputation as one of the very best up-and-coming single-seater drivers in the world.
“His summer test for McLaren was also remarkably impressive, and was further proof that he is ready to step up and take on a role of this size, scope and responsibility.
“At McLaren, we are all extremely keen to help him achieve his goal of racing in Formula 1.”