Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease primarily affecting motor neurons. Recent clinical evidence points towards cardiac dysfunction as a contributing factor to ALS-related mortality. The C9ORF72 hexanucleotide repeat expansion is the most common genetic cause of familial ALS, yet the effect of this mutation on human cardiomyocytes remains poorly understood. In this study, we investigated human cardiomyocytes (iCM) derived from induced pluripotent stem cells (iPSCs) of ALS patients carrying the hexanucleotide repeat expansion (HRE) mutation in the C9ORF72 gene (ALS^C9ORF72) in comparison to the healthy and isogenic lines. We identified hallmark ALS pathologies, including TDP-43 cytoplasmic mislocalization, elevated oxidative stress levels, and pATM-mediated DNA damage response in ALS^C9ORF72 iCMs. Moreover, compared to healthy and isogenic controls, functional analysis revealed significantly increased contraction frequencies and arrhythmic events in ALS^C9ORF72 iCMs. Our findings revealed for the first time that C9ORF72 mutation-carrying human cardiomyocytes display ALS hallmark pathologies and functional abnormalities in vitro, shedding light on the cell-autonomous mechanisms underlying ALS-related cardiac dysfunction in patients.