Agent Erik Burkhardt made the announcement, which was later confirmed by the team.
Hartley, a career 81.7 percent kicker in seven NFL seasons, received a one-year deal, sources told ESPN. Hartley spent six seasons with the Saints and a two-game stint with Cleveland, where he went 3-for-3 last year.
Suisham, one of the league's most proficient kickers last year after making 29 of 32 attempts, tore his ACL on a kickoff in Sunday's preseason opener against Minnesota. He will undergo surgery on his left knee this week and is likely to be placed on injured reserve, ending his season.
Steelers guard David DeCastro said the offense's mindset shouldn't change, with or without Suisham.
"You're always trying to score in the red zone -- if you're thinking about kicking field goals, that's not really the right mindset," he said.
The Steelers' preseason continues at Jacksonville on Friday.
Quarterback/wide receiver Devin Gardner was placed on the waived/injured list to make room on the roster for Hartley.