LaLiga will report a Rayo Vallecano fan that poked Sevilla winger Lucas Ocampos in the backside to the juvenile prosecutor's office.
The incident happened in the 33rd minute of Sevilla's 2-1 league win away at Rayo on Monday when Ocampos was ready to take a throw-in.
Video footage showed a fan in the stands poke his finger into Ocampos' backside, and the Argentine immediately turned around and confronted the three young fans sitting behind him before notifying the referee.
Sevilla and Rayo condemned the incident, which was included in the referee report, while Ocampos called on LaLiga to take action. The police have already identified the fan, who is a minor and hence will be reported to a juvenile court.
Spain's players' Union (AFE) said the incident was "shameful and regrettable" and "must be punished."
"We would like to place on record our disgust at the incident," Sevilla said in a statement. "Lucas Ocampos was subjected to an obscene and completely inappropriate act by a home fan. We hope that the appropriate measures outlined in the regulations will be taken to prevent such behaviour from happening again on a football field, and we have expressed this to LaLiga directly."
Rayo Vallecano said in a statement the incident went against their values as a football club and that they were working to identify the fan.
Ocampos said he wanted the league to treat the incident "as seriously as it takes racism and these things."
"You can see what happened in the [video] images," Ocampos told broadcaster DAZN. "I hope La Liga takes it as seriously as it takes racism and these things. I don't think all the Rayo people are like that because they have always treated us with respect, but there is always one fool.
"I hope it doesn't happen in other areas, because if this happens in a women's football match we know what can happen. I kept my calm because I have two daughters, I hope this doesn't happen to them in the future.
"I hope that the action that has to be taken is taken, and I hope that a fool like this does not stain a set of fans who otherwise behaved very well."
Information from the Associated Press contributed to this report