Stewart opted out of his contract with the Padres on Saturday, Robert Murray of The Athletic reports.
The 37-year-old had played 19 games for Triple-A El Paso, hitting .277/.333/.354. Stewart has appeared in at least one big-league game in 12 of the last 13 seasons but might be at the end of the road at this point, as he hasn't hit above .214 in the majors since 2015.