Hartson signed a reserve/futures contract with the Kansas City Chiefs in January but the team cut ties with him Thursday.
Hartson will try to unseat Warren, something a slew of younger long snappers have failed to do before him.
The Steelers re-signed Warren to a one-year, $1.05 million contract in February and the deal includes an $80,000 signing bonus. Warren only counts $665,000 against the Steelers' salary cap since it is a veteran minimum deal.
Warren has been the Steelers' long snapper since 2005. He is one of only four players left from the teams that played in three Super Bowls and won two of them from 2005-10. Quarterback Ben Roethlisberger, tight end Heath Miller and outside linebacker Jamres Harrison are the others.
The Steelers have two long snappers and three punters on their roster.