Valenzuela, 19, will be classified as a "young designated player" during the 2018 campaign. To open up a designated player slot for Valenzuela, the Crew have used targeted allocation money to pay a portion of Jonathan Mensah's contract and no longer make him a DP.
A 5-foot-7 left back, Valenzuela made two starts for Argentina at last year's U-20 World Cup in South Korea. He has made 13 Primera Division appearances since debuting for the club in 2016.
"Milton is a talented, young player who has a high pedigree at both the professional and youth international level," Crew coach Gregg Berhalter said in a news release. "His skill set has allowed him to gain critical experience playing in one of the continent’s most competitive leagues while featuring prominently for Argentina’s Under-20 national team.
"Milton has the character and determination to succeed in MLS and we are excited to welcome him to Columbus Crew SC, where we expect him to make an immediate impact."
Valenzuela, who joins Federico Higuain and Pedro Santos as a DP for the Crew, is the only natural left back on the Columbus roster. Earlier this offseason, the Crew traded Waylon Francis to the Seattle Sounders and saw Jukka Raitala sent to the Montreal Impact after being selected by Los Angeles FC in the expansion draft.
"MLS gets better year after year and good players continue to go there," Valenzuela told the Crew's official website. "In the last few years I think the league has become more attractive — I've watched a few games and I've enjoyed the soccer. Last season I followed Columbus Crew SC closely and they have a great team. I think that helped me decide to join the group."