Cullen tested positive as well as his manager David Moyes, and fellow player Issa Diop.
Irish international Josh Cullen has tested positive for Covid-19.
A statement posted on West Ham's website on Tuesday evening confirmed that Cullen, along with defender Issa Diop and manager David Moyes, had all returned positive tests for the virus.
West Ham's medical team were informed of the test results prior to their Carabao Cup game with Hull City, when the team were already at the London Stadium. Moyes and the two players left immediately, and went home to self-isolate.
All three are said to be asymptomatic.
Cullen (24), who was born in England, has been capped twice for the Republic of Ireland senior team, and has represented the country at underage levels as well.
Somewhat strangely, West Ham's game against Hull has gone ahead tonight.
West Ham said: "The Club’s measures and protocols around COVID-19 remain stringent. This included offering to test the Hull City team ahead of tonight’s fixture – an offer which they opted not to accept."
This comes hours after Spurs had their game against Leyton Orient cancelled, after a number of positive tests emerged from the League Two side.