The 20-year-old previously spent time on loan in the Midlands with Championship side Birmingham City last term, making 10 appearances as the side ultimately avoided the drop into the third tier on the final day of the season.
Bielik joined the Gunners from Legia Warsaw in 2015 and has made two senior appearances for Arsene Wenger's side to date - both in the League Cup.
Saddlers manager Jon Whitney said: "Krystian is a very technical, ball-playing centre-half and he's had a good loan spell with Birmingham last season, so he's got a Midlands connection. He was really good for them but he's had a shoulder injury in the last few months.
"I've been chasing him for about two or three weeks now, and we've persuaded him to come here ahead of other League One clubs and some abroad, too.
"We'd like to thank Arsenal and Arsene Wenger for allowing him to come to us."
Bielik joins Walsall with the club 18th in the League One table, just three points above the relegation zone.