Magnussen is ninth in the driver standings with 49 points in his second season with the American team following his move from Renault.
Grosjean, who has driven for Haas since its inception in 2016, has 27 points to his name, leaving them fifth in the constructor standings ahead of the Russian Grand Prix this weekend.
Team principal Guenther Steiner said the duo deserved to keep their seats following what has been Haas' best season.
"From the very beginning, we needed experienced drivers to hasten the development of our car and our team, and we have two very good and experienced drivers in Romain Grosjean and Kevin Magnussen," said Steiner.
"We've improved drastically from last year to this year, so retaining both drivers was a pretty easy decision."
NEWS: Haas F1 Team will retain its driver lineup of @RGrosjean and @KevinMagnussen for the 2019 FIA Formula One World Championship.
— Haas F1 Team (@HaasF1Team) September 28, 2018
He added: "I think the 100-point barrier should be and will be broken, and we'll do it with Romain and Kevin.
"Both drivers push each other to be better, and their collective feedback allows us to be better. It's not a coincidence that in their second year as team-mates, Haas is having its best year.
"By keeping Romain and Kevin together, we aim to do even better next year."Published 28 Sep 2018, 13:38 IST