The 22-year-old previously played with Polish side Pdobeskidzie Bielsko-Biala and made 21 league appearances for the club last season.
Nowak could go straight into the Hearts squad for the Betfred Cup tie at St Johnstone tonight.
Head coach Robbie Neilson said: “I’m very pleased to welcome Krystian to the club. I’ve had the opportunity to watch him in training and I have been very impressed by what I’ve seen.
“It was important to strengthen defensively following Jordan McGhee’s loan move and I feel Krystian will be a quality addition to the squad.”