"We are aware of the allegations and are now in the process of looking into it," MLB spokesman Mike Teevan said in a statement.
In a tweet posted Thursday, photographer Betsy Bissen said Sano grabbed her wrist and tried to pull her inside a bathroom at a store in a mall after an autograph session and that the 6-foot-4, 260-pound Sano tried to kiss her several times. ESPN does not typically identify people who make accusations of sexual assault unless they have chosen to go public with their names.
In her post, Bissen described how Sano flirted with her without reciprocation during the autograph signing. She said Sano's agent and her former boss were also at the event. She said Sano later grabbed her wrist and made her accompany him to an Apple store in the mall before trying to pull her into a restroom against her will.
"Every time he did, I said no and kept pulling back," Bissen said in her Twitter post. "... I was in a squatted position with my wrist throbbing. I screamed, no one came to help me. He finally gave up after a solid ten mins of fighting to pull me thru that door.
"... Every time I have to hear how great people think Miguel Sano is, I'm reminded of how awful he actually is and how he hurt me."
Sano issued a statement Thursday denying the allegations.
"I unequivocally deny the allegation made against me today -- it never happened," Sano said in the statement. "I have the utmost respect for women, especially those working in professional sports, and I deeply sympathize with anyone who has experienced sexual harassment. There is no place for it in our society."
Rob Plummer, Sano's former agent, confirmed he was at the mall for the signing but was not in a position to see the alleged incident.
"I was outside next to the car, waiting for him to come out on the other side of the loading bay dock, so I don't know what happened inside,'' Plummer told ESPN. "I didn't see or hear anything. I wasn't anywhere near Miguel.''
In replies to the tweet, Bissen said the Twins knew nothing about the incident at the time.
"Today the Minnesota Twins were made aware of allegations involving Miguel Sano at an offsite appearance during the 2015 season," the team said in a statement released on Thursday. "The Twins, along with Major League Baseball, take these allegations very seriously. Until more information is gathered, the Twins will have no further comments."
Bissen said that she told people, including her family, at the time, but that she wasn't quite ready to share her story more widely until Thursday. She used the hashtag #metoo with her tweet, which has gained popularity in encouraging women to publicize experiences to demonstrate the widespread nature of sexual assault.
She said she now works for the St. Paul Saints, an independent baseball team.
Sano, 24, signed with the Twins out of his native Dominican Republic in 2009 for a bonus of $3.15 million. He finished third in American League Rookie of the Year balloting in 2015. This past season, he hit 28 home runs, drove in 77 runs and made the All-Star team for the first time.
MLB and the players' association reached agreement on a domestic violence, sexual assault and child abuse policy in August 2015. The policy allows commissioner Rob Manfred to decide on appropriate discipline, with no minimum or maximum penalty. Players have the right to challenge any punishment before an arbitration panel.