Voters passed Amendment 9 with 75 percent of the vote.
The passage means law enforcement will now need a search warrant to look through text messages and e-mails.
St. Joseph state Senator Rob Schaaf helped write the amendment.
He says the vote sends a strong message about the right for privacy.
"Today's overwhelming support for Amendment 9 reflects Missourians emotions that they feel about the erosion of their electronic privacy. People are upset and they spoke with a loud voice," said Schaaf.
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