The two guards received all 100 first-place votes from a panel of sports writers and broadcasters. Both players helped their teams reach the second round of the playoffs.
Boston's Jayson Tatum also was selected Tuesday to the first team, falling one vote short of unanimous. Simmons, Mitchell and Tatum are the three finalists for the NBA's Rookie of the Year award.
Celtics coach Brad Stevens said Tuesday that "somebody made a mistake" when asked how Tatum missed out on being a unanimous selection by one vote.
The NBA noted that the individual balloting of voters won't be revealed until after its award show following the season.
"He's had an unbelievable rookie year," Stevens said of Tatum. "I know that any awards that come his way is well-deserving."
Lakers forward Kyle Kuzma (93 first-team votes) and Chicago's Lauri Markkanen (76) rounded out the first team.
"This is one of the best draft classes in the past 10-15 years, or they say since '03," Kuzma said. "Just a credit to the level of talent that has come into this league and really took it by storm and everything."
The second team was guards Dennis Smith Jr. of Dallas and Lonzo Ball of the Lakers, Atlanta forward John Collins, Sacramento guard Bogdan Bogdanovic and Phoenix forward Josh Jackson.
Information from The Associated Press, ESPN's Chris Forsberg and ESPN's Ohm Youngmisuk was used in this report.
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