A club statement described the incident as “sickening”.
West Brom keeper Ben Foster branded the fan a “disgrace” while winger James McClean called the culprit “cowardly”.
Albion boss Tony Pulis said: “For someone to throw a coin at a player of the football club he’s supporting – the game’s gone mad.
“The world’s gone mad if people accept that.”
The ugly scenes overshadowed a wonderful day for Championship Reading, who fought back from 1-0 down to claim a place in the quarter-finals.
After a dull first half, Darren Fletcher gave West Brom the lead nine minutes into the second, finishing well after a neat move.