Walker, 25, has spent most of his time in the minors. He appeared in 13 games for Baltimore from 2014-15, going 4 for 27 with a solo homer. The Braves claimed him off waivers last month.
To make space on their 40-man roster, the Reds designated reliever Jumbo Diaz for assignment. Diaz, 33, has pitched for the Reds each of the last three seasons. He went 1-1 with a 3.14 ERA in 45 games last year.