The Reds selected Hopkins' contract from Triple-A Louisville on Saturday.
With TJ Friedl (hamstring) landing on the 10-day injured list Saturday, Hopkins will join the Reds' active roster to bolster the club's outfield depth. The 26-year-old outfielder has been impressive in Louisville this season, slashing .341/.437/.540 with seven home runs and 27 RBI through 207 plate appearances. To make room for Hopkins on the 40-man roster, Nick Lodolo (leg) was transferred to the 60-day injured list.