Two players tested positive upon their arrival at the Disney resort in Orlando on June 27 ahead of the MLS is Back Tournament, which begins in July.
It was discovered two days later that four others had also contracted the disease at the resort, according to the league.
FC Dallas said that all members of the club received negative test results before their departure for Florida on June 27.
"Upon receipt of the positive COVID-19 test, all six players immediately participated in a clinical assessment by a healthcare provider and were moved to the isolation area of the hotel," read a statement from the league.
"While in isolation, the individuals are in daily communication with and receive remote care from a health care provider, including monitoring of symptoms and regular follow-up testing."
Members of FC Dallas' delegation "will remain quarantined in their hotel rooms pending the results of further COVID-19 testing."
The MLS is Back Tournament will take place between July 8 and Aug. 11.