He had one interception and two passes defensed in 12 games (two starts) last season. He has 27 interceptions and 114 passes defensed in 133 career games.
San Francisco worked out Hall twice since June before signing him.
The 49ers, who are depleted in the secondary after Sunday's 26-23 loss to the Indianapolis Colts, also claimed defensive back Dexter McCoil off waivers from the Los Angeles Chargers on Monday and waived safety Lorenzo Jerome. The team is hopeful that safety Eric Reid can return from a knee injury this week.
Ahkello Witherspoon suffered a concussion during the Colts game. Fellow cornerback Asa Jackson left the game because of a hamstring injury and did not return. To make room for Hall on the roster, Jackson was placed on injured reserve Tuesday.
The winless Niners (0-5) will visit Washington on Sunday.
ESPN's Nick Wagoner contributed to this report.