The pair will join defending champion Karolina Pliskova of the Czech Republic and top ranked Australian Ashleigh Barty in the women's field for the 10th edition of the tournament, which is sponsored by Suncorp.
World number one Rafael Nadal of Spain and world number three Andy Murray of Great Britain are confirmed on the men's side.
"I'm really looking forward to returning to Brisbane this summer," says Muguruza, who will make her third consecutive appearance.
"Brisbane was the start of a successful season for me in 2017 and it was an easy decision to play again next year."
After winning her second Grand Slam title at Wimbledon, Muguruza rose to world number one in September and was named the 2017 WTA Player of the Year. After her most successful year on tour, Muguruza's attention now turns to the Australian summer of tennis.
"The conditions are perfect leading into the Australian Open; the weather, the stadium and the strong competition all help when you are preparing for the first Grand Slam of the year," Muguruza says.
American Stephens, the 2017 US Open champion and world number 15, will compete in Brisbane for the first time.
"2017 was a real game changer for me, and I want to continue that momentum into next year, starting with Brisbane and the Australian Open," says Stephens.
"As players you are always looking to play events that prepare you for the big stage - the Grand Slams and I've heard Brisbane is the perfect preparation for Melbourne.
"I'm really excited to be coming to Brisbane this summer, I've heard such wonderful things about the tournament and the city and I have to come and see what it's all about."
Tournament Director Geoff Quinlan is excited to have two current Grand Slam champions in the women's draw.
"Garbine and Sloane are both incredibly exciting players who will thrill fans in Brisbane with their brilliant shot-making," Quinlan said.
"Muguruza rose to world number one this year and is such a consistent player on the women's tour, while Stephens' story is nothing short of incredible; going from considering giving the game away to becoming a Grand Slam champion in the space of six months."
(Image courtesy: Tennis Photo Network)
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