Antonio Rudiger was allegedly racially abused during Real Madrid's Club World Cup victory over Mexican side Pachuca on Sunday, manager Xabi Alonso said.
Alonso confirmed a complaint had been made after an incident towards the end of the Spanish giants' 3-1 victory in Charlotte.
Rudiger had clashed with Pachuca captain Gustavo Cabral in injury time but it was not clear if the alleged abuse came from a player or someone in the crowd.
"That's what Rudiger said, and we believe him," Alonso told reporters in his post-match press conference. "It is important to have zero tolerance in these kinds of situations. FIFA now is investigating. That's all I can say."
FIFA, the world governing body and tournament organiser, has not commented but referee Ramon Abatti had crossed his arms in front of his chest, which signals the activation of the anti-racism protocol.
Real had shrugged off the early sending off of Raul Asencio to claim a victory that took them to the top of Group H.
Asencio was dismissed in the seventh minute for hauling down Salomon Rondon - another blow for Alonso's side with striker Kylian Mbappe still missing through illness.
But after soaking up a spell of pressure, Real broke the deadlock through Jude Bellingham in the 35th minute before Arda Guler doubled the lead.
Federico Valverde volleyed in a third with 20 minutes left before the Mexicans grabbed a late consolation through Elias Montiel's deflected effort.
It was Real's first win of the tournament and took them level on four points with Red Bull Salzburg, who played out a goalless draw with Al Hilal in Washington DC.
Real play Salzburg in their final game of the first stage on Thursday night (2am Friday UK) but, with Al Hilal just two points behind, the group is wide open.
