The 26-year-old is the latest fresh face to join Michael Flynn's new-look squad and arrives having helped Bolton win promotion from League Two in 2020-21.
He had a spell at Dagenham & Redbridge in the National League last season and boasts 16 caps for Montserrat, who Saddlers defender Donervon Daniels has also represented.
"I’m excited and I can’t wait to get going,” said Comley.
“I spoke to the head coach and and he is expecting an exciting season and he is hopeful that we can do good things this year.
“I am not new to the league but every year is different, so I am just going to get my head down, work and try to give the fans and the gaffer everything I can offer, whether that is experience in the league or whether that is hard work and sheer determination just to do well for the club.
“I am over the moon to get the deal done and I just can’t wait to get going now.”
Comley, a product of the QPR academy, played 88 times for Colchester before joining Bolton.
He left the Trotters last November before spending the second half of 2021-22 with Dagenham.
Walsall boss Flynn added: "Brandon is a great signing for this football club.
“I’ve always liked him, he’s at a good age and has experience of getting promoted out of the league.
“He’s played higher, he’s a current international and I’m delighted to get him.
“Brandon’s very good technically on the ball and I can’t wait for him to get going.”