Riddle had his contract selected by the Reds on Tuesday.
The 30-year-old signed with Cincinnati on a minor-league deal in mid-March and will join the big-league roster as infield depth with Jonathan India (hamstring) and Mike Moustakas (biceps) heading to the injured list. Riddle appeared in only four major-league games with the Twins last year and had a .591 OPS at the Triple-A level.