Kostecki will join fellow rookie Will Brown in what will be an all-new line-up for the squad.
Like Brown, Kostecki has first-hand experience with the squad, after partnering Anton De Pasquale in the #99 entry at the Bathurst 1000 this year.
Kostecki will continue in the #99 for his full-time promotion, which means Brown will take over the #9 from Reynolds.
“It’s very exciting for me and for the team as well, we are all really looking forward to going racing next year,” said Kostecki.
“I’ve been fortunate enough to be with the team for a while now and I think that’s a great thing going into next year.
“Though my first year in Supercars it will be my second year with the team, and we are all looking forward to making the next step together.
“Will and I will definitely push each other hard as we’re both very hungry and eager to get results.
“We are already part of the family and that’s really important; we all have a good time, but we are all still hungry to get results.”
Erebus CEO Barry Ryan is confident that Kostecki is ready to make the step from the second-tier Super2 series.
“Brodie is a hard-arse racer and exactly what we need in our team,” Ryan said.
“His experience in America is something many people don’t understand and as a racer he is no rookie.
"Brodie is more than ready and both he and Will are far beyond where we expect any young driver to be.
“We are really excited to have two young but familiar faces pushing each other, and if we can give them the car then they can certainly deliver the results.”
The pair will use their rookie test day next month before the official pre-season test at Sydney Motorsport Park in February.