The 28-year-old has agreed a two-year deal at the Banks’s Stadium and will officially join on Thursday, when his contract at Harrogate Town expires.
Kiernan spent the past two seasons in Yorkshire and was part of the team which helped Harrogate win promotion to the EFL in 2020.
Last season he scored six goals in 35 League Two appearances, including a hat-trick in a 5-4 win over Cambridge in April.
Previously on the books of Welling United, Hampton & Richmond, Braintree and Bromley among others, he now joins a Walsall team looking to challenge this season in League Two under new boss Matt Taylor.
“I’m looking forward to joining the club and I’m excited about being here,” Kiernan said.
“I can’t wait to meet the boys and it was great to meet the head coach as well.
“I’m looking to bring energy, hard work, chip in with some goals and create chances for the team.”
Taylor has been busy revamping his playing squad since being appointed Darrell Clarke’s successor last month, with Kiernan joining Conor Wilkinson, Manny Monthe, Joss Labadie and Jack Earing on the books.
“Brendan is an exciting player who we are extremely happy to have at the club,” said Taylor.
“He’s somebody I feel we can utilise his strengths but also add to that.
“He’s desperate to succeed and as with all of our previous signings, his ambition matches ours and we can’t wait to start working with him.”